— Cheered by the biggest crowds of his campaign, Republican Mitt Romney declared Sunday that 42-year-old running mate Paul Ryan is ready to be president, but said his own budget plan, not the more detailed proposals of his partner, will be the basis of his White House bid.

"I have my budget plan," he said. "And that's the budget plan we're going to run on."

Earlier, Romney walked a careful line as he campaigned with Ryan by his side in North Carolina, singling out his running mate's work "to make sure we can save Medicare." But the presidential candidate never said whether he embraced Ryan's austere plan himself, and he addressed the matter more directly in a "60 Minutes" interview, with Ryan still with him, that aired Sunday night on CBS.

Democrats weren't about to let them off that hook.

President Barack Obama, attending campaign fundraisers Sunday in Chicago, tagged Ryan as the "ideological leader" of the Republican Party.

"He is a decent man, he is a family man, he is an articulate spokesman for Gov. Romney's vision but it is a vision that I fundamentally disagree with," Obama said in his first public comments about Ryan's selection.

Senior Obama adviser David Axelrod and other aides spent Sunday trying to brand Ryan's budget "the Ryan-Romney plan."

During the Republican primary, Romney had called Ryan's budget a "bold and exciting effort" that was "very much needed."

Ryan proposed to reshape the long-standing entitlement by setting up a voucher-like system to let future retirees shop for private health coverage or choose the traditional program — a plan that independent budget analysts say would probably mean smaller increases in benefits than current law would provide.

Romney and Ryan, in their first joint television interview Sunday, were clearly mindful that some of Ryan's proposals don't sit well with key constituencies, among them seniors in critical states like Florida and Ohio. Each man sought to reassure older voters they wouldn't take away their benefits, with Ryan saying his mother is "a Medicare senior in Florida" and Romney vowing there would be "no changes" for seniors currently counting on the popular federal program.

"In America, the nature of this country has been giving people more freedom, more choices," Romney said. "That's how we make Medicare work down the road."

Romney praised his running mate for his policy depth and analytical skills and said if they should win the election, Ryan will surely be consulted in big decisions — "along with other individuals." He added: "Obviously I have to make the final call in important decisions."

The presumptive presidential nominee said Ryan, "if it were necessary, could become president." And Romney extolled his running mate's Washington experience, despite having criticized primary rivals Newt Gingrich and Rick Santorum for their years in the nation's capital.

Ryan said he planned to release two years of personal tax returns to the public. The wealthy Romney is also releasing two years of returns, despite pressure from Democrats and some Republicans to provide more information about how he manages his millions.

Romney's selection of Ryan has jolted the presidential contest, until now one that had done little to draw the public's attention, and set the contours for the fall campaign: Romney as a proponent of a friendlier business climate seeking to revitalize the economy and rein in federal spending and Obama casting himself as a defender of middle-class families and federal spending on health care, retirement pensions and education.

The running mate pick also shifted the campaign debate, at least temporarily, to the pressing economic challenges facing the country — a debate both Romney and Obama have said they wanted to have even as the dialogue had spiraled into nasty, personal attacks. Sunday was a marked departure from the previous week, when the race for the White House devolved into name-calling and accusations of lying from both campaigns.

Three months from Election Day, polls find Obama with a narrow lead over Romney, though the race remains tight in key battleground states. And while Ryan's selection raised the role of government spending and Medicare in the election, the fundamentals of the campaign remained unchanged: a race defined by a weak economy and high unemployment, measured most recently at 8.3 percent in July.

Romney, seeking to pull his campaign out of a summer slump, appeared to relish in campaigning alongside the youthful and energetic Ryan.

"This is Day Two for me," a gleeful Romney told a campaign rally in Moorseville, N.C. "This is Day Two on our comeback tour to get America strong again, to rebuild the promise of America." He meant a comeback for the country, but that could apply as well to his campaign.

The duo blitzed through North Carolina — a competitive battleground state in the November election — as part of a multistate bus tour. The team made two campaign stops in North Carolina Sunday – at 10 a.m. at the NASCAR Technical Institute in Mooresville and at 1 p.m. in High Point – and the presidential hopeful laid out a five-step plan he says will create 12 million jobs.

Romney pulled out of a planned campaign event at Smokey's Shack, a barbecue restaurant at 10800 Chapel Hill Road in Morrisville, but the candidate's son, Matt Romney, filled in as headliner. Pat McCrory, the Republican candidate for governor, was also on hand.

The pair ended the day in Waukesha, Wis., with a homecoming-themed event for Ryan, who was in tears as he took the stage.

Romney, emboldened by the enthusiastic crowds that greeted the pair Sunday, wrapped up a day of campaigning with a sharp shot at the tone of Obama's campaign. "Mr. President, take your campaign out of the gutter," he said.

Romney then planned to head to Florida and Ohio as the week begins, while Ryan was scheduled to travel to Iowa on Monday as the ticket looked to cover as much ground as possible.

For Ryan, the weekend of campaigning was a chance to make a first impression on many voters. A recent CNN/ORC international poll found a majority of voters had no opinion of the congressman, an up and comer in Washington but far from a household name. Nearly 40 percent had never heard of him and 16 percent weren't sure what they thought of him.

The 42-year-old congressman embraced the attack dog role traditionally assumed by the No. 2 on the ticket. He said Obama had turned his 2008 campaign slogan of "hope and change" into "attack and blame."

"We're not going to fall for it," Ryan told a crowd of 5,000 in High Point, N.C.

Obama's campaign had already been trying to tie Romney to Ryan's tough budget blueprint even before the Wisconsin congressman emerged as a contender for the GOP ticket. Democrats believe seniors, those nearing retirement and middle-income voters will view Ryan's long-term budget plan remaking Medicare and cutting trillions in federal spending as a threat to their financial security.

Campaign officials were readying state-specific strategies aimed at seniors in Florida and Ohio, and also planned to court young people and military service members who they believe will be turned off by other elements of Ryan's proposed budget cuts.

As chairman of the House Budget Committee, Ryan is the primary author of conservative tax and spending proposals that the tea party-infused Republican majority approved over vigorous Democratic opposition in 2011 and again in 2012.

They envision transforming Medicare into a program in which future seniors would receive government checks that they could use to purchase health insurance. Under the current program, the government directly pays doctors, hospitals and other health care providers.

Ryan and other supporters say the change is needed to prevent the program from financial calamity. Critics argue it would impose ever-increasing costs on seniors.

Other elements of the budget plan would cut projected spending for Medicaid, which provides health care for the poor, as well as food stamps, student loans and other social programs that Obama and Democrats have pledged to defend.

In all, it projects spending cuts of $5.3 trillion over a decade and would cut future projected deficits substantially. Romney, too, has proposed ambitious cuts in federal spending, but without the specifics that make Ryan's plan so attractive to fiscal conservatives and such a target for Democrats.

Republicans say Ryan could help put Wisconsin, which traditionally has voted Democratic in presidential campaigns, in play and that the Catholic Midwesterner also could appeal to blue-collar voters whom Romney, a Mormon and multimillionaire, has struggled to reach in Iowa and elsewhere.

Obama's campaign had no plans to start running new television ads in Wisconsin following Ryan's pick. Officials said they didn't think Ryan was popular enough statewide to swing Wisconsin toward the Republican ticket.

Obama's campaign argues Ryan's budget could be a powerful campaign tool for the president in states like Pennsylvania and Iowa, in addition to Florida and Ohio.

Down ballot, party leaders hoped to work in tandem with Obama to turn the Ryan budget into a litmus test in congressional races, forcing Republican opponents to take ownership of the plan. The campaign arm of the House Democrats, for example, was urging its lawmakers to call Ryan's budget plan — not the man himself — Romney's new "running mate."

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Thomas reported from Chicago. Associated Press writer Julie Pace contributed from Washington.