A CNN poll of the U.S. vice-presidential debate watchers released following Thursday’s matchup between Rep. Paul Ryan and Vice President Joe Biden showed 48% of respondents named Ryan the winner and 44% said Biden won.
The margin between the two candidates was within the poll’s five point sampling error.
A poll taken immediately after last week’s first presidential debate showed a much more decisive victory for Mitt Romney.
Sixty seven percent of debate watchers questioned said that the Republican nominee won the faceoff, with one in four saying that President Barack Obama was victorious.
US vice-presidential candidates Joe Biden and Paul Ryan have begun their one and only debate, as polling show the US election race tightening.
Vice-President Biden and Mr Ryan, a Wisconsin congressman, will clash for 90 minutes at a college in Kentucky.
Democrats hoped to change the narrative after what was widely seen as a poor debate performance by President Barack Obama last week.
Mr Obama said on Wednesday he had been "too polite" to his rival, Mitt Romney.
The Obama campaign has since accused Mr Romney, a Republican former business star and Massachusetts governor, of shifting his policy positions to appeal to moderate voters and of lying during their meeting last week in Denver, Colorado.
Thursday's debate is being moderated by Martha Raddatz, senior foreign affairs correspondent for ABC News, and will cover both domestic and foreign policy.
It is being held at Centre College, a small liberal arts university about 129km from the city of Louisville
U.S. Vice President Joe Biden and his Republican challenger Paul Ryan locked horns over the deadly attack on the U.S. consulate in Libya's Benghazi immediately after their debate began.
Picking up the Romney campaign's usual tone, Ryan slammed the Obama administration's weakness on foreign policy, saying it took two weeks for President Barack Obama to declare the Libya incident as a terror attack.
Biden criticized the Romney-Ryan ticket for making political statements even before they got to know more facts about the incident, which left the U.S. ambassador and three others killed.
Vice presidential debates traditionally do not carry the same influence as presidential debates in elections. But stakes have been raised for both campaigns over the 90-minute debate on Thursday evening.
After Obama's lackluster performance in the first presidential debate with Republican nominee Mitt Romney last week, Biden is under pressure to do well in this debate.
U.S. Vice President Joe Biden defended the way that President Barack Obama was handling the Iranian nuclear stalemate on Thursday.
In the vice-presidential debate in Danville, Kentucky, Biden argued that Mitt Romney was offering an Iran policy which was not so different from that of Obama.
He also stressed that if deemed necessary, the U.S. military was ready to deal a serious blow to Iran's nuclear ambition.
U.S. Republican vice presidential nominee Paul Ryan said Thursday said Republican presidential candidate Mitt Romney and him agree on the administration's plan to transfer security in 2014 to the Afghan side.
Ryan made the comments during the vice presidential debate with Vice President Joe Biden at Centre College in Danville, Kentucky.
He said although the Republican ticket agree with the administration's timeline to draw down forces in Afghanistan, but "we don't want to lose our gains ... we want to see 2014 transition successful," and want to give the generals what they need to make it successful.
Biden, on the other hand, stressed the U.S. side will end the war in 2014, and transfer the security lead to the Afghan side by the end of that year.