Democrats "haven't put a credible solution on the table," he said. "They'll tell you about vouchers. They'll say all these things to try to scare people."
Biden quickly said that Ryan had authored not one but two proposals in which seniors would be given government payments that might not cover the entirety of their care. Otherwise, he said, the Romney-Ryan approach wouldn't achieve the savings they claimed.
Unlike Obama, Biden had no qualms about launching a personal attack on Romney.
After Ryan argued that Romney's plan would pay for reduced tax rates by eliminating tax loopholes for the wealthy, Biden noted that on a recent interview on CBS' "60 Minutes," Romney defended the 14 percent tax rate he pays on his $20 million income as fair, even though it's a lower rate than some lower income taxpayers pay.
"You think these guys are going to go out there and cut those loopholes?" Biden asked, addressing the national TV audience, his tone of voice indicating he did not.
But Ryan said he and Romney believe "taking 28 percent of families' and businesses' income is enough."
"What we're saying is lower tax rates across the board and close loopholes primarily on the higher income people," Ryan said. He said that instead of specifying what loopholes and other tax breaks would be eliminated, Romney preferred to lay out broad principles in hopes of reaching a bipartisan agreement.
Across 90 minutes, the two men agreed precisely once.
That was when Ryan, referring to the war in Afghanistan, said the calendar was the same each year. Biden agreed to that, but not to his rival's underlying point, which was that it was a mistake for Obama to have announced a date for the withdrawal of the remainder of the U.S. combat troops.
The fiercest clash over foreign policy came in the debate's opening moments, when Ryan cited events across the Middle East as well as Stevens' death in Libya as evidence that the administration's foreign policy was unraveling. The Republican also said the administration had failed to give Stevens the same level of protection as the U.S. ambassador in Paris receives.
Biden rebutted by saying that the budget that Ryan authored as chairman of the House Budget Committee had cut the administration's funding request for diplomatic security by $300 million.
On the nation's economy, both men were asked directly when his side could reduce unemployment to 6 percent from the current 7.8 percent. Both men sidestepped.
Biden repeated the president's contention that the nation is moving in the right direction, while Ryan stated the Republican view that economic struggle persists even though Democrats had control of both houses of Congress during the first two years of Obama's term.
"Where are the 5 million green jobs" we were told would be created? Ryan said to Biden.
Obama campaigned in Florida during the day. Mocking recent changes in Romney's rhetoric, he told a rally in Miami rally, "After running for more than a year in which he called himself severely conservative, Mitt Romney is trying to convince you that he was severely kidding."
Romney visited with 93-year-old Billy Graham in North Carolina before speaking to an evening rally in Asheville, N.C. "Prayer is the most helpful thing you can do for me," he told the evangelist.
For Biden, Thursday night's debate was his first since the 2008 campaign, when he shared a stage with Sarah Palin, then John McCain's running mate.
Ryan spars frequently with Democrats during debates on legislation on the House floor and in the House Budget Committee, which he chairs, but not in a one-on-one encounter covering 90 minutes and a virtually unlimited range of topics.
For all their differences, the two men shared a common objective, to advance the cause of their tickets in a close race for the presidency. And they appeared to avoid any gaffes that might forever seal their place in the history of debates.
Associated Press writers Philip Elliott in Kentucky, Ken Thomas in Florida and Kasie Hunt in North Carolina contributed. Espo reported from Washington.