(CNN) -- On a relatively quiet Friday on the campaign trail, President Barack Obama's team grabbed the attention with moves that spotlighted Republican challenger Mitt Romney's tax returns and his Medicare proposal.

With neither candidate making any public appearances, their advisers and surrogates kept up attacks in the contentious battle to frame the candidates and major issues less than three months before the November election.

Obama's campaign manager, Jim Messina, fired the first shot by making pubic a letter to his counterpart in the Romney camp that offered a "deal" in which the former Massachusetts governor would release five more years of past tax filings in return for Democrats agreeing to drop the issue.

"If the governor will release five years of returns, I commit in turn that we will not criticize him for not releasing more -- neither in ads nor in other public communications or commentary for the rest of the campaign," Messina's letter to Romney campaign manager Duncan Rhoades.

His offer did not include a promise to avoid attacking Romney on the contents of the released returns, or make any commitments for independent third party groups.

Rhoades rejected the offer.

"It is clear that President Obama wants nothing more than to talk about Governor Romney's tax returns instead of the issues that matter to voters, like putting Americans back to work, fixing the economy and reining in spending," Rhoades wrote in response to Messina.

Romney has released his return for 2010 and an estimate for 2011. He has pledged to release the full 2011 return once it is completed by his accountant, which must occur by October 15 under the extension he filed with the Internal Revenue Service in April.

Democrats criticize Romney for not releasing more tax details, arguing that voters are entitled to a greater amount of disclosure from candidates running for president.

At the White House on Friday, Deputy Press Secretary Josh Earnest responded to repeated questions on the issue by noting previous presidential candidates released as many as 12 years of tax returns under a precedent set by Romney's father, former Michigan Gov. George Romney, in his unsuccessful presidential bid.

"All the questions you are asking could be answered before the end of the day today, but that will require Governor Romney to live up to a standard that's been met by every other major-party presidential candidate since his father ran for president in 1968," Earnest said.

Some Republicans also have called on Romney to release more of his returns, saying the political headache could be easily ended by simply putting out additional information.

Romney and his campaign have consistently rebuffed such calls, saying the two years released so far go beyond the legal requirement and offer voters a good perspective into the candidate's financial background. They also say Democrats will launch unfair attacks on any further tax information released by Romney, who is a multimillionaire.

Questioned again Thursday about his tax returns, Romney revealed more details than he had previously, telling reporters that he paid no less that 13% of his income in taxes for the past decade while reiterating his refusal to make those returns public.

That did not satisfy Obama's senior campaign adviser, David Axelrod.

"I don't accept his word on what his taxes say," Axelrod told CNN on Thursday night. "Let the public see it. Let them understand what's in those tax returns. They say, 'well, we don't want to do it because the opposition may make issues about it.' Well, that doesn't give me much confidence about what's in those tax returns."

Republican Rep. Jason Chaffetz of Utah, a Romney surrogate, told CNN on Friday that the tax return issue was "a sidebar attempt to distract from the real issues that are out there."

"They did two years required by law and they did four years that was done as the governor of Massachusetts, so I think it's kind of a side issue that's just crazy," Chaffetz said.

A CNN/ORC Poll taken August 7-8 showed 63% of voters thought Romney should release additional years of tax returns. Among independents, 67% said the Republican candidate should release more years of returns.

Meanwhile, the Obama campaign released a new television advertisement on Medicare intended to rebut the Romney campaign's attacks on the issue this week, particularly a claim discounted by independent sources that Obama's health care reform cuts $716 billion from the government health care program for senior citizens.

In the ad titled "Facts," the Obama campaign quotes AARP, an advocacy group for senior citizens, as saying Obama's health care reforms passed in 2010 crack down on Medicare fraud, waste and abuse while strengthening guaranteed benefits.

It also challenges the proposal by Romney's running mate, House Budget Committee Chairman Paul Ryan, to partially privatize Medicare for people currently younger than 55, saying it could result in higher costs for retirees.

The Romney campaign responded immediately, with spokeswoman Andrea Saul reiterating Romney's claim that Obama cut Medicare by $716 billion, including money for enhanced coverage called Medicare Advantage. In addition, Saul said in a statement, Medicare will run out of money in 2024 under Obama's reforms.

At the White House, Earnest noted that the Obama reforms in the 2010 Affordable Care Act extended the solvency of Medicare for eight years, while the pledge by Romney to repeal the law known by critics as "Obamacare" would leave the program to go broke by 2016.

The Medicare issue became a major campaign topic last week when Romney chose Ryan as his running mate. Knowing the Obama team would raise the politically sensitive Medicare issue, Romney's campaign attacked first in an attempt to blunt a perceived advantage for Democrats.

It was unclear how the debate would impact the election, especially in states like Florida that have large populations of senior citizens, said CNN senior political analyst David Gergen.

"That's what gives suspense to this campaign," Gergen said Thursday night.

Any candidate who calls for Medicare reforms that could take away existing benefits is "going to pay a price, and there are some polls coming out of Florida now suggesting Obama has been helped by the Ryan selection," he added.

The issue might be causing a divide between different age groups, according to Gergen.

"What I also found interesting in the CNN poll in Wisconsin is that among 65 and older, Romney has a substantial lead over President Obama," he said. "But with the crowd from 50 to 64, it's Obama who has a substantial lead. There's a 14-point swing from the 65-year-old types. And it's the 50 to 64-year-old folks who could be affected by Medicare reform."

Ryan made no mention of his Medicare plan in a campaign speech in Virginia, one of his two appearances in the battleground state on Friday. He repeated the Romney pledge to repeal "Obamacare" in remarks that focused on the need for leadership to address the nation's mounting federal deficit and debt.

The Wisconsin congressman also accused Obama of hindering small business growth through what Republicans contend are excessive regulations and high taxes.

"We should not have a government that stands in the way, that erects barriers to small business," Ryan said, calling the opportunity to build an idea into a successful business the essence of "what it is to be an American."

"You see, when another American succeeds that's not a bad thing," Ryan added. "It's a good thing. We take pride in their success. We don't resent success."

The Romney campaign said Friday it received more than $10 million in online donations since the August 11 announcement of Ryan as running mate.

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CNN's Tom Cohen, Shawna Shepherd, Rachel Streitfeld, Adam Aigner-Treworgy, Peter Hamby, Gregory Wallace and Kevin Liptak contributed to this report.