St. Louis, Missouri (CNN) -- In his first major speech to conservatives since his Republican rival Rick Santorum quit the presidential race, Mitt Romney turned his sights on the general election and accused President Barack Obama of waging "an assault on our freedoms."
Romney addressed the Annual Meeting of the National Rifle Association, which drew more than 60,000 gun enthusiasts to St. Louis to inspect the latest in personal weaponry and feast on tough political attacks against the White House.
"This November, we face a defining decision," Romney told a large crowd on the floor inside Edward Jones Dome, the home of the St. Louis Rams.
"I am offering a real choice and a new beginning," he said. "I am running for president because I have the experience and the vision to lead us in a different direction. We know what Barack Obama's vision of America is. We've lived it this last 3½ years. Mine is very different."
Romney advisers billed the NRA address as the first in a series of speeches from the likely Republican nominee designed to shift the discussion away from a divisive primary fight and steer the political discussion toward the general election battle against Obama.
While Second Amendment rights were on the agenda -- Romney praised NRA officials and promised to stand up for the rights of gun owners -- the likely Republican nominee entered the arena with larger ambitions.
One senior Romney aide called Friday's speech "the beginning of crystallizing for voters the choice that they are going to have in the fall" between Romney and "a big-government liberal."
Repeatedly invoking the Constitution and the Declaration of Independence, Romney framed the 2012 election as a choice between his vision of limited government and the president's heavy-handed approach toward taxes and regulation.
He also accused Obama of impinging on religious liberty by requiring faith-based employers to provide contraception in their insurance plans.
"We've seen enough of President Obama over the last three years to know that we don't need another four," Romney said. "In a second term, he would be unrestrained by the demands of re-election."
The Obama campaign painted Romney as a hypocrite and highlighted the slow rate of Massachusetts job growth and fee increases under his leadership.
"This is the governing record we have to judge Mitt Romney on and it's one that he can neither rewrite nor erase," said Obama spokeswoman Lis Smith.
Romney advisers are hoping an economy-themed, anti-Obama message will appeal to both the swing voters who will be decisive in November, and a Republican base that has been slow to warm to Romney but remains scornful of the president.
By choosing to focus on a big picture vision of his administration, Romney was able to avoid a deep dive into gun issues, which have provided for some awkward moments during his political career.
During his 1994 Senate race, Romney said: "I don't line up with the NRA."
In his first presidential bid, he falsely claimed that he had been endorsed by the NRA and was later forced to backtrack.
Romney was roundly mocked in that race for claiming to be a lifelong hunter of "small varmints" like rabbits and rodents.
NRA members who watched the speech seemed not to care about Romney's record on guns, nor did they complain that he might not have been their first choice in the Republican race.
Romney checked the most important box: He is not Barack Obama.
"There is no other choice but Romney," said Jim Greger, 74, an Illinois retiree. "I would vote for Donald Duck before I'd vote for Obama."
Lillian Lamonato, 75, a Romney supporter from South Carolina, said winning in November is more important than ideological purity.
"He is conservative enough," she said of Romney. "He had to do what he had to do up there in Massachusetts. Nobody is perfect."
NRA President Wayne LaPierre revved up the crowd before the Romney speech and implored his group's 4 million members to get to work unseating Obama.
"We know if President Obama gets a second term, America as we know it will be on it's way to being lost forever," LaPierre said to lusty applause.
Other big name Republicans addressed the NRA meeting, including Santorum, Texas Gov. Rick Perry, Louisiana Gov. Bobby Jindal and former House Speaker Newt Gingrich, who continues to run for the GOP nomination despite long odds.
"This is still a more open nominating process than anyone in the elite media believes," Gingrich argued.