Vice President Joe Biden launched a new line of attacks on Mitt Romney and Paul Ryan on Friday, dismissing the Republican vice presidential nominee as "that guy" and asking an audience who it would believe on the critical issue of Medicare.

Romney's campaign described the attacks as false and warned they would backfire.

Biden's charges came on the first of a two-day swing in Florida. The vice president rallied supporters at two events Friday, in Boca Raton and Tamarac. Both events were well attended by seniors - a crucial voting bloc in Florida who, polls show, favor Romney over President Barack Obama.

Biden, who will attend a Saturday morning rally in Fort Myers, Florida, told his audiences that Romney's economic plan could hit seniors with more taxes to their Social Security benefits.

"The beneficiaries, who will be getting in the future, will be getting considerably less in Social Security payment," the vice president said.

"Instead of dealing with a balanced approach, how to keep these programs in place, they've decided what you do is you shift all the costs to the recipients. As the president pointed out, their plan on Social Security, the one they have now, would raise taxes on your social security."

Biden continued: "Right now, the majority of seniors over 50 pay zero income tax on their social security benefit. You have another group that pays no more than half - income tax on half of that. And you have another group that pays income tax on 15%. Well, if Governor Romney's plan goes into effect, it could mean that everyone, every one of you would be paying more taxes on your Social Security. The average senior would have to pay $460 a year more in taxes for their Social Security."

The Romney campaign responded quickly to Biden's claims.

"Vice President Biden is using Social Security to fabricate the Obama campaign's latest false attacks," spokesman Ryan Williams said in a statement. "However, these attacks will backfire when voters learn he has repeatedly supported higher Social Security taxes, and that seniors face a 25% across-the-board benefit cut because of President Obama's failure to lead on this issue."

"Mitt Romney and Paul Ryan have a plan to save and strengthen Social Security that does not raise taxes and ensures that our middle-class seniors receive all of the benefits they've earned."

The Romney campaign referred reporters to a 1993 vote Biden cast over a budget resolution that raised taxes on seniors making as little as $34,000.

Meanwhile, Biden continued his recent assault on his opponents' plans for Medicare.

At one point, the vice president chided reporters. It happened as the vice president spoke about Ryan's first budget proposal revealed in the spring of 2011.

"(The Congressional Budget Office) said the plan, the first plan - that the press somehow thinks it's unfair for us to talk about - would raise the cost for every senior by $6,400 a year," Biden said.

"Well, the press accurately points out, they accurately say, 'You know what? They changed that plan. So why do you keep bringing it up?' Well the reason I bring it up, I think it's fair to bring it up. If Congressman Ryan felt so strongly about insisting that this plan become law, and if Gov. Romney said he would've signed it into law ... then it goes to their motive. It goes to who you believe."

"So now they tell you that this new plan they have isn't going to cost seniors any money," Biden said. "Who are you going to believe? Are you going to believe somebody who was willing and knowingly to increase the cost to you, for your Medicare by $6,400 a year and now is willing to increase your cost if you're 50 years old when the, by the time you'll be eligible by $60,000? Are you going to believe the guy - that guy - when he says, 'No, no, no, no. I'm not going to affect Medicare at all.' Or are you going to believe me?"