What is going on?

JUDY WOODRUFF: We turn to the presidential campaign, as the candidates sparred over Medicare and the Republican nominee offered up some long-awaited financial information.

MITT ROMNEY (R): We got work to do! Thank you!

JUDY WOODRUFF: Mitt Romney addressed a long-running campaign issue, releasing his 2011 tax returns, showing that Romney and his wife, Ann, earned $13.7 million last year, mostly investment income, and paid just over $1.9 million in taxes, for an effective tax rate of 14.1 percent.

They also donated a little more than $4 million to charity in 2011, almost 30 percent of their income. Romney didn't comment on his taxes at a stop in Las Vegas late today.

His campaign also posted online a letter from accountants saying he paid an average effective federal tax rate of 20 percent over a 20-year period ending in 2009.

MITT ROMNEY: This is a time of choice for the country. This is a time when we can decide to go down the path President Obama has put us on. He said he wants to pool our resources and reallocate. All right? He's going to take from some and give to others.

This redistribution idea -- this redistribution idea has been tried in other places. This is not a new idea. It's just never worked in other places.

JUDY WOODRUFF: President Obama has criticized Romney for refusing to release more than two years of returns.

And appearing via satellite before a conference of the American Association of Retired Persons this morning, Mr. Obama brought up Romney's secretly recorded remarks about Americans who don't pay income taxes, some of whom are elderly people on Social Security.

PRESIDENT BARACK OBAMA: Given the conversations that have been out there in the political arena lately, I want to emphasize Medicare and Social Security are not handouts. You have paid into these programs your whole life.

(APPLAUSE)

BARACK OBAMA: You have earned them. And, as president, it's my job to make sure that Medicare and Social Security remain strong for today's seniors and for future generations

JUDY WOODRUFF: Both campaigns set out to woo older voters today, critical because they are almost twice as likely to vote as the youngest voters.

Republican vice presidential nominee Paul Ryan made the Romney campaign's pitch to the AARP in person, drawing some boos when he insisted the president's health care law harms Medicare.

REP. PAUL RYAN, R-Wis.: The first step to a stronger Medicare is to repeal Obamacare, because it represents the worst of both worlds.

(BOOING)

REP. PAUL RYAN: I had a feeling there would be mixed reaction, so let me get into it.

(BOOING)

REP. PAUL RYAN: It weakens Medicare for today's seniors and puts it at risk for the next generation.

JUDY WOODRUFF: Debate over Medicare has intensified since Ryan joined the ticket. Democrats charge, his budget plan would end the program.

NARRATOR: It's true. Mitt Romney would replace Medicare's guaranteed benefits with a voucher system.

JUDY WOODRUFF: A new Obama ad makes that case, accusing Romney of seeking to raise seniors' costs by up to $6,400 a year.

That's currently airing in Colorado, Iowa, and Florida. And at an event today in Virginia, the president said he would fight any changes to the program.

BARACK OBAMA: I will refuse to turn Medicare into a voucher.

(CHEERING AND APPLAUSE)

BARACK OBAMA: Americans who have worked hard shouldn't have to spend their golden years at the mercy of insurance companies.

(CHEERING AND APPLAUSE)

JUDY WOODRUFF: For its part, the Romney campaign will continue its pitch to older voters this weekend, as Paul Ryan heads to all-important Florida, which boasts the largest proportion of seniors in the nation.