The race for president shifted dramatically Friday into a general election matchup between President Obama and Republican Mitt Romney as the candidates delivered dueling, sharp-tongued speeches about the president’s leadership.

In Appleton, Wis., Romney assumed the mantle of presumptive nominee, delivering a revamped address attacking what he called Obama's vision for a “government-centered society.” Obama, in a four-stop campaign swing through New England, offered a robust defense of his first term and invoked his 2008 rallying cry by characterizing his actions as “what change is.”

For months, the Republican nominating contest has dominated national headlines, with Romney fending off intense challenges from more conservative alternatives. But with the former Massachusetts governor pulling ahead in the delegate count and the White House moving into full reelection mode, both sides are now treating the campaign as a duel between Romney and Obama.

The president took credit for saving the auto industry, preventing a historic economic collapse and passing a sweeping health-care overhaul (though he did not mention the Supreme Court case challenging the law). His work is unfinished in Washington, he said, and he exhorted wildly enthusiastic crowds in deep-blue Burlington, Vt., and suburban Portland to help him win another term — so that more change can come.

Obama also signaled the broad themes in his quest for reelection and offered his sharpest critique yet of what he called the GOP’s vision for America. Although he never named Romney, he framed a choice between middle-class security and “you’re on your own” economics. He rebuked his detractors for claiming the edge in “values.” He portrayed Republicans as more extreme and less willing to seek bipartisan solutions than ever. And he embraced his accomplishments, even the controversial ones, with a new vigor.

“I warned you in the campaign this was going to be hard,” Obama told a packed arena at Southern Maine Community College, where his speech was punctuated by shrieks of support. “Big change is hard. It takes time. It takes more than a single year. It takes more than a single term. It takes more than a single president.”

Romney, in Wisconsin, delivered the latest of what his campaign calls his “framing speeches,” this one about “restoring America’s promise.” It was a passionate defense of America’s free-enterprise system, which he said has been under attack by an administration that considers business as “the villain and not the solution.”

“In Barack Obama’s government-centered society, the government must do more because the economy is doomed to do less,” Romney said. “When you attack business and vilify success, you will have less business and less success.”

Romney, stopping short of labeling Obama a socialist, saying: “President Obama is transforming America into something very different than the land of the free and the land of opportunity. And we know where that transformation leads. There are other nations that have chosen that path and it leads to chronic high unemployment, crushing debt and stagnant wages. Sound familiar?”

Romney was introduced by Rep. Paul Ryan (R-Wis.), the top House budget writer whose endorsement earlier in the day signaled the candidate’s latest success in convincing Republicans to rally around him.

The pairing highlighted Romney’s efforts to position himself as a deeply conservative candidate, as did his support for Ryan’s controversial House budget plan, which proposes cuts of $5.3 trillion over the next decade entirely through deep reductions in entitlements and agency spending. In his remarks, Romney rattled off a stark list of statistics, including the number of Americans in poverty, the rate of foreclosures, the number of car dealers shuttered, all of which he blamed primarily on the president.

Romney mocked Obama for saying he’s done a good job as president and suggested that the president is simply out of touch with the experience of most Americans.

If Romney hopes to win the White House by presenting a sharp contrast with its current occupant, he must also reckon with an opponent who appeared more than ready Friday to draw the contrast himself.

“In 2008, I was running against a candidate who believed in climate change, believed in immigration reform, believed in reducing deficits in a balanced way,” Obama said at a luncheon fundraiser in Burlington. “We had some profound disagreements, but the Republican candidate for president understood that some of these challenges required compromise and bipartisanship.”

He added: “And what we’ve witnessed lately is a fundamentally different vision of America and who we are.”

The luncheon was Obama’s first stop of the day. He went on to address a crowd of about 4,500 supporters at the University of Vermont, about 1,800 at Southern Maine Community College in South Portland and a more intimate fundraising dinner at the Portland Museum of Art.

Obama's New England swing marked his final campaign events before the end of the first-quarter fundraising period and brought into focus the fundraising advantage he has enjoyed. Tickets started at $7,500 for the Burlington luncheon and $5,000 for the art museum dinner; general admission to the larger events started at $100. Through the campaign’s “Gen44” program, however — a quest for financial support from smaller-dollar donors — a limited number of $44 tickets also were available.

Obama raised $45 million last month, a figure that dwarfed Romney’s $12 million, though the Obama figure includes money he raised for the Democratic National Committee.

Obama’s reelection campaign began ramping up two weeks ago, when Vice President Biden delivered the first of four speeches that advisers say are meant to frame the election around the issues and accomplishments that have most occupied the White House: the auto bailout, bolstering American manufacturing and contrasting the president’s budget with that of Ryan’s — particularly Ryan’s proposal to reduce Medicare benefits.

Obama’s trip was the first visit by a sitting president to Vermont since 1995, when then-President Bill Clinton traveled there.