Washington (CNN) -- Republican presidential candidate Mitt Romney told a conservative audience what it wanted to hear Friday, accusing President Barack Obama of leading the country away from the founding fathers' vision and "toward limited freedom and limited opportunity."
In an address to the National Rifle Association's convention billed by Romney's advisers as a launch of his general election campaign, Romney said Obama's attack on freedom is to blame for the country's slow economic recovery from recession.
"The Obama administration's assault on our economic freedom is the principal reason why the recovery has been so tepid -- why it couldn't meet their projections, let alone our expectations," Romney said.
While Romney remains short of the delegates needed to claim the Republican presidential nomination, his closest pursuer -- conservative challenger Rick Santorum -- dropped out of the race this week and the two other contenders -- former House Speaker Newt Gingrich and Texas Rep. Ron Paul -- have no realistic chance of winning.
A senior Romney adviser who briefed reporters on condition of anonymity characterized the Romney address as "the beginning of crystallizing for voters the choice that they are going to have in the fall between a big-government liberal, represented by Barack Obama, and somebody who believes in the freedoms which are an essential part of our Constitution, which are consistent with our founders, and that would be Mitt Romney."
Gingrich and Santorum followed Romney in addressing the NRA gathering, which served as a conservative conclave with other top Republicans -- including House Majority Leader Eric Cantor and House Oversight Committee Chairman Darrell Issa -- also scheduled to speak.
Santorum, who had rejected Romney's conservative credentials on the campaign trail, pledged his full commitment to helping conservative candidates win in November but didn't endorse Romney.
"We have to be all in and I pledge to you that even though I am no longer in this race that I will be all in between now and November," Santorum said. "I will do everything we can to make sure that we elect Republicans, conservatives up and down the ticket, and with your help we will make that happen."
Not surprisingly, both Santorum and Romney also offered NRA members strong support for the rights of gun owners, now under extra focus as the Trayvon Martin shooting resonates through the country.
Gingrich went even further in his remarks, calling for a U.N. treaty that extends the right to bear arms to every person on Earth.
Meanwhile, Obama was en route Friday to Cartagena, Colombia, for the sixth annual Summit of the Americas with leaders from the Western Hemisphere to discuss the economy, trade, energy and regional security.
Before leaving the United States, Obama stopped in the battleground state of Florida to talk trade and the economy, announcing a new program that he said would help businesses get financing and counseling on how to access new markets south of the border.
"This initiative is going to help our small business: Latino-owned businesses. Women-owned businesses. African-American-owned businesses," Obama said, hitting on a few key political constituencies.
Earlier Friday, the White House released Obama's tax returns, which showed that the first family's income dropped by almost $1 million in 2011 as sales from his best-selling books slowed.
The Obama family reported an adjusted gross income of $789,674 for 2011, down from $1,728,096 in the previous year. The family income topped $5.5 million in 2009.
Obama earns a salary of $400,000 for his day job. His total earnings are much higher because his books -- "Dreams from My Father," and "The Audacity of Hope" -- continue to attract buyers.
The Obamas reported paying $162,074 in federal taxes. They donated $172,130, or 22% of their income, to 39 different charities.
Jay Carney, the White House press secretary, noted in a statement that Obama's 2011 effective federal income tax rate is 20.5% and that the president's own policies would result in higher taxes for the Obama family.
In its own statement Friday, the Obama campaign challenged Romney to release his returns for past years, saying Romney "has yet to provide tax returns from the period in which he made hundreds of millions as a corporate buyout specialist, or as governor of Massachusetts, the experience he says qualifies him to be president."
"Mitt Romney's defiance of decades of precedent set by presidential candidates on both sides of the aisle, including his own father, begs the question -- what does he have to hide?" the campaign statement said, citing disclosure earlier this year that Romney had a Swiss bank account and established a corporation in Bermuda.
Romney campaign spokeswoman Andrea Saul said the candidate filed an extension for his 2011 return on Friday.
"Sometime in the next six months, and prior to the election, Gov. Romney will file and release the 2011 return when there is sufficient information to provide an accurate return," Saul's statement said, adding that Romney had filed similar extensions in previous years.
Her statement made no mention of the Democratic call for Romney to file returns from previous years.
Romney's tax returns previously arose as an issue in January, when Democrats, along with fellow Republican presidential candidates, pressured Romney's campaign to release the documents.
On January 24, Romney's team released his 2010 tax return, as well as an estimate for 2011. Opponents quickly derided the disclosure as paltry, noting that other candidates, including Romney's father during his run for the GOP nomination in 1968, released a decade's worth of tax returns.
The forms released then showed Romney earned $42.7 million over the past two years and paid $6.2 million in taxes. Over the two years, Romney's effective tax rate -- the percentage of his income that he owed in federal income taxes -- was 14.5%.
Democrats, eager to align themselves with middle-class voters, quickly assailed Romney for paying a low tax rate while earning millions of dollars.
Obama is pushing for Congress to approve the so-called Buffett Rule, which would ensure that Americans making more than $1 million a year pay at least 30% in taxes.
On Thursday, Vice President Joe Biden slammed Romney at a campaign stop in New Hampshire for promoting a "Romney Rule" on taxes that would benefit only millionaires like him -- a harsh comparison with the "Buffett Rule" that Democrats are promoting.
"The Buffett Rule says that multimillionaires should pay at least the same percentage of their income in taxes as middle-class families do," Biden said Thursday. "The Romney Rule says the very wealthy should keep the tax cuts and loopholes they have, and get an additional, new tax cut every year that is worth more than what the average middle-class family makes in an entire year."
In response, Saul called the Democratic attacks a diversionary tactic.
"It's no surprise with the worst job creation record in modern history that President Obama would try to distract Americans from the real issues with a series of sideshows," she said.
Campaign rhetoric kicked into high gear this week once Santorum suspended his bid for the GOP nomination.
On Thursday, a Democratic strategist apologized for a comment questioning Ann Romney's qualifications to advise her husband on women's economic issues, while the Romney campaign sought to exploit the controversy to help fix a gender gap problem in the race against Obama.
Hilary Rosen issued a statement after fellow Democrats, including a Twitter post on Michelle Obama's page, criticized her remarks the night before on CNN.
"I apologize to Ann Romney and anyone else who was offended," said the statement by Rosen, who is a CNN contributor. "Let's declare peace in this phony war and go back to focus on the substance."
In a later interview on CNN, Rosen said that she "should not have chosen words that seemed to attack Ann Romney's choice in life" and that she hoped Ann Romney "understands I didn't mean it personally," adding "I was trying to talk about economic issues."
She also said the controversy over her remarks portended "an ugly campaign season."
The political firestorm ignited quickly after Rosen said Wednesday night on CNN's "AC360˚" that Mitt Romney shouldn't be relying on his wife for guidance on economic issues affecting women.
"What you have is Mitt Romney running around the country, saying, 'Well, you know, my wife tells me that what women really care about are economic issues, and when I listen to my wife, that's what I'm hearing,' " Rosen said. "Guess what? His wife has actually never worked a day in her life."
Republicans slammed Rosen's comments as disparaging to stay-at-home mothers, and top Democrats, including Obama and his chief campaign adviser, also chimed in.
On Thursday, Twitter posts attributed to Michelle Obama and Democratic National Committee head Rep. Debbie Wasserman Schultz took exception with Rosen's remarks.
"Every mother works hard, and every woman deserves to be respected," the first lady tweeted.
Ann Romney told Fox News earlier Thursday that raising five boys was hard work, and she hears from women all the time about economic difficulties they face.
"Look, I know what it is like to struggle," said Romney, a cancer survivor who has multiple sclerosis. "If maybe I haven't struggled as much financially as some people have, I can tell you and promise you that I have had struggles in my life. And I would love to have people understand that Mitt and I have compassion for people that are struggling, and that's why we are running. We care about those people that are struggling, and we recognize that this economic recovery has been very weak."
With polls repeatedly showing female voters favoring Obama over the former Massachusetts governor, Romney launched a harsh attack Wednesday on how the administration's economic policies hurt women.
Rosen's comments provided another opening on the issue for the Romney campaign.
Republican Rep. Cynthia Lummis of Wyoming said Rosen was a frequent White House visitor who was saying what the Obama campaign intended.
"Clearly they are using surrogate women, including Hilary Rosen, who is a paid spokesperson, to deliver messages about Republicans that the president does not want to deliver himself for fear of the backlash," she said.
Rosen said she is not an adviser to the White House or the Democratic National Committee.
At the White House, Carney said Thursday he was unaware of how many times Rosen had visited. In a sign of the sensitivity of the issue, Carney repeatedly emphasized Obama's policies on women, such as making the Lilly Ledbetter Fair Pay Restoration Act the first piece of legislation he signed into law.
Romney's campaign still struggles to generate enthusiasm among the GOP conservative base, which questions his more moderate stances as Massachusetts governor. In addition, a Washington Post/ABC News poll this week affirmed findings of other recent polls that Romney trails Obama among female voters.
Another poll released Thursday by Quinnipiac University showed Obama leading Romney in New Jersey, even under the hypothetical scenario that popular Republican Gov. Chris Christie was on the GOP ticket as Romney's running mate.
New Jersey is considered a solidly Democratic state in presidential general elections. The Quinnipiac poll also showed a Romney-Christie ticket trailing against Obama and Biden among female voters.
The Republican presidential campaign has included a conservative shift to appeal to tea party voters in the primary and caucus season. However, some socially conservative polices opposing abortion and health care coverage for contraception appear to be raising concerns among female voters.
Democrats have seized on that dynamic by emphasizing Republican stances that they say harm equal treatment and opportunity for women. In response, the Romney campaign is targeting Obama's economic policies as being bad for women.
Romney got a boost Thursday when two prominent anti-abortion groups -- the Susan B. Anthony List and National Right to Life -- endorsed his nomination, a sign that conservatives will coalesce around his candidacy despite concerns about his more moderate history as Massachusetts governor.
CNN's latest estimate of the GOP delegate tally shows Romney with 659, Santorum with 275, Gingrich with 140 and Paul with 71. It takes 1,144 delegates to clinch the nomination.
By suspending his campaign, Santorum can continue raising money and likely keep control of some or all of his delegates.
Both Gingrich and Paul say they intend to stay in the race to the GOP convention in August.
The next primaries are April 24 in New York, Connecticut, Rhode Island, Pennsylvania and Delaware, with 231 delegates at stake.
The goal now for Gingrich and Paul is to prevent Romney from reaching the 1,144-delegate threshold before the convention.
CNN's Tom Cohen, Kevin Liptak, Jim Acosta and Paul Steinhauser and CNNMoney's Jeanne Sahadi and Charles Riley contributed to this report.