Washington (CNN) -- Mitt Romney, fresh off a three-primary sweep that put him more than halfway to the delegate number needed for the Republican presidential nomination, gets the chance Wednesday to answer President Barack Obama's harsh attacks the day before against Republican ideology.
Romney will address a convention of news editors, the same group that heard Obama on Tuesday frame the November presidential election as a choice between his administration's efforts to boost the middle class versus Republican policies that he said favor the rich.
The primary victories Tuesday in Wisconsin, Maryland and Washington, D.C., padded Romney's wide lead in the delgate count over top challenger Rick Santorum and bolstered the sense that Romney's nomination is inevitable.
Conservative Sen. John McCain of Arizona said as much Wednesday, telling CNN's "Starting Point with Soldedad O'Brien" that "whether Rick Santorum stays in or not is now basically irrelevant."
Ari Fleischer, a CNN contributor and former George W. Bush White House spokesman, said that Santorum earned the right to fight through Pennsylvania, but the race was all but over.
"I think the writing is on the wall. Does Rick Satorum want to read it is the question," Fleischer said.
Santorum said Tuesday night he intended to remain in the race, despite the increasingly long odds against a comeback victory.
The former U.S. senator from Pennsylvania is pegging his hopes on a home state victory in the next set of primaries on April 24 to rebuild momentum for a campaign based on capturing conservative support by contrasting his principles and record against the more moderate Romney.
"Pennsylvania and half the other people in this country have yet to be heard, and we're going to go out and campaign here and across this nation to make sure that their voices are heard in the next few months," Santorum told supporters in appealing for Republicans to back a strong conservative to run against Obama.
"Time and time again, the Republican establishment and aristocracy have shoved down the throats of the Republican Party and people across this country moderate Republicans because, of course, we have to win by getting people in the middle," Santorum said, adding: "We win by getting people to the middle to move to us and move this country forward."
According to CNN estimates, Romney now has 654 delegates to 270 for Santorum, while fellow contenders Newt Gingrich and Texas Rep. Ron Paul trail well back. Romney's total is 57% of the 1,144 needed to clinch the nomination, and he would have to win 44% of the remaining delegates available to reach that threshold.
In a speech to supporters in Milwaukee Tuesday night, Romney made no mention of his GOP rivals and instead mostly contrasted himself with Obama.
"We won a great victory tonight in our campaign to restore the promise of America," Romney said before criticizing Obama's economic polices and what he repeatedly called the president's plan for a "government centered society."
"There is a basic choice that we're going to face: The president has pledged to transform America, and he spent the last four years laying the foundation for a new government-centered society," Romney said. "I will spend the next four years rebuilding the foundation of an opportunity society led by free people and free enterprises."
Obama, meanwhile, clinched the Democratic presidential nomination Tuesday with primary wins in Maryland, the District of Columbia and Wisconsin, CNN projected. Unlike the Republicans, Obama faced no serious opposition in his race and his nomination was never in doubt.
On Tuesday, Obama delivered what amounted to his strongest campaign speech so far, building on themes of equal opportunity from a speech in Kansas last year and the State of the Union address in January.
The president referred to Romney by name for the first time in a speech, signaling he now identified the former Massachusetts governor as his likely opponent in the November election.
In attacking a Republican budget plan passed by the U.S. House of Representatives, Obama said that "one of my potential opponents, Gov. Romney, has said that he hoped a similar version of this plan from last year would be introduced on day one of his presidency" and that Romney was "very supportive of this new budget."
To Obama, the Republican proposal prepared by House Budget Committee Chairman Paul Ryan of Wisconsin is a "Trojan Horse" disguised as a deficit reduction plan that actually imposes a "radical vision" far to the right of the party's mainstream heroes such as Ronald Reagan.
"It is thinly-veiled Social Darwinism," Obama said. "It is antithetical to our entire history as a land of opportunity and upward mobility for everyone who's willing to work for it -- a place where prosperity doesn't trickle down from the top, but grows outward from the heart of the middle class."
He added that "by gutting the very things we need to grow an economy that's built to last -- education and training; research and development; infrastructure -- it's a prescription for decline."
Obama blamed a polarized political climate for an inability to make progress on such key issues as deficit reduction and entitlement reform, arguing that the Republican shift to the right rejected moderate proposals acceptable to Democrats.
"The problem right now is not the technical means to solve it. The problem is our politics, and that's part of what this election and what this debate will need to be about," Obama said. "Are we, as a country, willing to get back to commonsense, balanced, fair solutions that encourage our long-term economic growth and stabilize our budget?"
He took particular aim at the Ryan budget proposal's tax reforms, spending cuts and entitlement reforms, which Obama said would shift the burden of deficit reduction to the middle class, senior citizens and the poor.
"In this country, broad-based prosperity has never trickled down from the success of a wealthy few," Obama said. "It has always come from the success of a strong and growing middle class."
Romney responded to Obama's speech later Tuesday, telling conservative commentator Sean Hannity's radio program that the president "says things which are designed to solicit applause and affirmation, but in reality, he does very different things than what he's saying."
"It's a disingenuous, fear-mongered approach, which I understand is going to catch a lot of attention," Romney said. "But if people dig a little deeper, I think they're going to understand that this is President Obama being President Obama, which is finding a way to deflect blame and to mischaracterize the efforts on the part of very thoughtful and serious minded individuals."
The $3.5 trillion Ryan budget plan for 2013 plan would lower tax rates and cut spending while reforming the Medicare and Medicaid government-run health care programs for senior citizens, the disabled and the poor.
Ryan and other conservatives argue that major reforms are needed to subdue increasing federal deficits and debt, with particular focus on entitlement programs that are the main contributors to the budget imbalance.
For example, the Ryan budget would convert federal Medicaid funding for states into block grants. Such a step could increase the cost burden on states, but would give them more autonomy about how to set up their Medicaid programs. As a result, a state could reduce how many people are eligible or increase enrollees' cost-sharing obligations.
CNN's John Helton, Paul Steinhauser, Jason Hanna, Gabriella Schwarz, Phil Gast and Peter Hamby contributed to this report.