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Associated Press
Republican presidential candidate, former Massachusetts Gov. Mitt Romney, accompanied by Florida Attorney General Pam Bondi, right, and Sen. Marco Rubio, R-Fla., speaks to reporters on the tarmac at Miami International airport, Monday, Aug. 13, 2012, in Miami, Fla. (AP Photo/Mary Altaffer)

COUNCIL BLUFFS, Iowa — Led by President Barack Obama, Democrats claimed on Monday that Republican challenger Mitt Romney privately backs controversial plans to overhaul Medicare and cut trillions from social programs that his new vice presidential running mate has publicly proposed.

Rep. Paul Ryan "has given definition to the vague commitments that Romney has been making," Vice President Joe Biden said as the Democrats welcomed the Wisconsin lawmaker to the race with a barrage of criticism. "There is no distinction" between the two, he said.

Romney lauded his running mate's work as he resumed his own four-day bus trip through campaign battleground states.

Ryan has "come up with ideas that are very different than the president's," Romney said in Florida, the state with the highest percentage of residents age 65 and over. "The president's idea for Medicare was to cut it by $700 billion. That's not the right answer. We want to make sure that we preserve and protect Medicare."

The former Massachusetts governor did not say so, but the tax-and-spending plans Ryan produced in the past two years as chairman of the House Budget Committee call for the repeal of Obama's health care plan but also would retain the $700 billion in Medicare cuts that were part of it.

Aides said during the day that Romney has long disagreed with that specific element of the House Republican plan, and they said while he wants to repeal the health care law he also wants to restore the funds to Medicare.

The day marked a first in the race for the White House, with both major party tickets campaigning at full strength and a little more than 80 days remaining in a campaign dominated by a weak economic recovery and a national jobless rate of 8.3 percent.

Polls taken before Romney added Ryan to his ticket over the weekend showed Obama with a slender advantage in a contest that will be decided in eight to 10 battleground states.

Large crowds turned out for Romney and Ryan's joint appearances over the weekend, and conservatives have hailed the selection of the Wisconsin lawmaker, who is regarded by fellow Republicans as an intellectual leader within the party on budget issues.

Democrats counter that Ryan's presence on the ticket will make it easier to tag Romney with political ownership of the budgets that Republicans pushed through the House in 2011 and again this year. They say it will help their candidates in House and Senate races as well as the campaign for the White House.

As chairman of the House Budget Committee, Ryan was the chief architect of tax and spending plans that called for turning Medicare into a voucher-like program beginning a decade from now. Critics say it would lead to higher costs for beneficiaries, while supporters argue that a fundamental change is needed to prevent the program from going bankrupt.