Tim Garraty/CNN

President Barack Obama and GOP vice presidential nominee Paul Ryan on Friday traded sharp criticism over health care reforms and Medicare, with each telling a leading advocacy group for senior citizens that the other was being untruthful.

"Contrary to what you've heard and what you may hear from subsequent speakers, Obamacare actually strengthened Medicare," the president told the AARP Liffe@50+ event, using the nickname for the 2010 Affordable Care Act that passed with no Republican support.

In particular, he called the claim by Ryan and other Republicans that $716 billion is being cut from Medicare to fund the health care bill "simply not true."

Ryan spoke to the same event shortly afterward, saying that Obama's contention that the health care law strengthened Medicare was "just not true," adding that the legislation "turned Medicare into a piggy bank for Obamacare."

The debate over Medicare is a major issue in the November election campaign, especially in the vital battleground state of Florida with its large population of senior citizens.

Ryan headed to Florida later Friday, while Obama campaigned in Virginia, Vice President Joe Biden spoke in New Hampshire and GOP presidential nominee Mitt Romney headed to Las Vegas. All four states are considered up-for-grabs in the election less than seven weeks away.

In a separate development, Romney released details of his 2011 income tax return that showed he made $13.7 million last year and paid $1.94 million in federal taxes, giving him an effective tax rate of 14.1%, his campaign said.

The majority of the candidate's income last year came from his investments, Brad Malt, the trustee of Romney's blind trust, said in a blog post. The Romneys gave just over $4 million to charity.

In addition, the Romney campaign said his tax filings from 1990 to 2009 show that he and his wife paid 100% of the federal and state income taxes they owed and that their overall average annual effective federal tax rate was 20.2%.

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Obama defends health care reforms