Incumbent Vice President Joe Biden accused opponent Paul Ryan of "loose talk" when it came to the Republican vice-presidential candidate's statements on Iran when the two faced off in their first and only televised debate on Thursday.
"You think Iran's not brazen? Look at what they're doing," said Ryan, a congressman from Wisconsin ."They're stepping up their terrorist attacks.
"Look at where they are," he continued. "They're moving faster toward a nuclear weapon."
Biden fired back that Iran is "a good way away" from building a nuclear weapon.
"Facts matter," said Biden.
"All this loose talk about, 'all they have to do is to get to enrich uranium in a certain amount and they have a nuclear weapon'....Not. True," said Biden, adding that "action" will be taken if need be.
At the start of the debate, Ryan slammed President Barack Obama's administration for failing to call the attack on the US consulate in the Libyan city of Benghazi a terrorist attack.
Biden, in return, criticised Ryan and Republican candidate Mitt Romney for launching political attacks before they knew the facts on the ground.
Ryan said the US is witnessing the unravelling of Obama's foreign policy. He said the attack that killed US Ambassador Chris Stevens shows the US is projecting weakness abroad.
Biden said that's "a bunch of malarkey'.'
He said the US will bring those responsible to justice and ensure any mistakes are not repeated.
Al Jazeera's Patty Culhane, reporting from Danville, Kentucky, where the debate was taking place, said that for both candidate, the pressure is on.
"Traditionally, vice-presidential debates haven't mattered, really, at all," she said.
"But this election seems to be bucking most of the current trends."
After Obama's lackluster performance last week, Biden, 69, is expected to mount a full-throated attack against the surging Romney ticket on Thursday, while striving to avoid the gaffes the veteran politician is famous for.
The much younger Ryan, 42, whose controversial government-slashing budget made him a hero among conservatives, has never debated on a national stage.
Biden said Republicans must take responsibility for obstructing the economic recovery and that Ryan, the House Budget Committee chairman, has stood in the way of making middle-class tax cuts permanent and helping struggling home owners.
Ryan attacked the Obama administration's recovery plan, pointing out that unemployment in Biden's hometown of Scranton, Pa., is at 10 per cent, compared to 8.5 per cent when Biden and Obama took office.
"Joe Biden is a genuine populist. He's the one who can connect to the people the best"
- Bill Schneider, political analyst
Ryan said Biden and Obama want to raise taxes on small businesses, while he and Romney want to help struggling Americans get good jobs.
The vice president said Romney's opposition to the auto bailout and government steps to prevent foreclosures "shouldn't be surprising" given his comments about the 47 per cent of Americans who don't pay income tax.
Biden was referring to remarks Romney made to wealthy donors. In a secretly recorded video, Romney said 47 per cent of Americans believe they are victims and entitled to government help.
The vice president said some of those people are senior citizens living off social security. Obama never mentioned Romney's comments in his first debate, to the dismay of many Democrats.
Romney has since said his comments were wrong. Biden, who frequently burst out laughing during the debate, said that if voters believe they were a mistake, he has "a bridge to sell you.''
On domestic issues, the access to abortion also divided Biden and Ryan, both of whom are Catholics and have spoken of their faith repeatedly.
Ryan said that with the exceptions of rape, incest and when the life of the mother is in danger, a Romney administration would oppose abortion.
Biden said he did not wish to impose his religious beliefs on others and that as far as he is concerned, the decision to seek an abortion was "between a woman and her doctor".
US troops abroad
Regardring the withdrawal of US troops from Afghanistan, Biden said it is up to Afghanistan to take responsibility for its own security.
Ryan said he does not want the US to lose the gains achieved in its decade-long war there following the September 11, 2001, terrorist attacks.
Both Biden and Ryan conveyed that it is time to wind down US involvement, with Ryan agreeing with Obama in transitioning out of the country by 2014.
But the vice-presidential candidate said the White House should not announce a deadline for withdrawal and expose weakness.
Political analyst Bill Schneider told Al Jazeera that Ryan needs to be ready for Biden's attack.
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Schneider said he expected Biden to attack Ryan on Medicare.
"Paul Ryan wants to change Medicare to a voucher programme. You can bet that Joe Biden is going to talk about that repeatedly," he said.
"Joe Biden is a genuine populist. He's the one who can connect to the people the best."
The vice-presidential debate rarely elicits much excitement, but this year all eyes will be on Danville, to see whether Biden can stem Romney's sharp rise in the polls over recent days.
Republican candidate Mitt Romney's climb in polls since Obama's poor showing in their first debate last week has intensified expectations for the vice-presidential showdown with less than four weeks before the November 6 election.
Schneider said that that Obama "did not show any fight" in the debate.
"Mitt Romney got down and dirty and [Obama] didn't defend himself. And that really demoralised his party," said Schneider.
Recent polls show an unsettled race, with some national polls - like Gallup's daily tracking survey which had Romney and Obama tied at 48 per cent - suggesting Romney's debate bounce was subsiding.
A Fox News poll had Romney up a single point. Obama had led the same poll by five points before the debate.
There was also movement towards Romney in state surveys that had the race in battlegrounds like Nevada, Florida, Nevada and Ohio within a few points.