U.S. Vice President Joe Biden is about to square off against the Republican presidential challenger's running mate, Representative Paul Ryan, in their only scheduled debate.

The nationally televised event at a small college in Kentucky comes as Republican presidential nominee Mitt Romney is gaining momentum in voter opinion polls after his strong performance in last week's debate against the Democratic incumbent, President Barack Obama.

The 69-year-old Biden is widely regarded as an experienced debater and skilled politician, based on his 36 years in the Senate before becoming vice president in 2009. He is expected to help the campaign overcome Mr. Obama's lackluster debate performance. But Biden is also known for making occasional embarrassing gaffes, or misstatements, due to his exuberant nature.

Ryan, a 42-year-old congressman from Wisconsin, is considered a rising star among conservative Republicans. But he has far less experience on the national stage than Biden, having served in Congress for just 14 years.

The presidential contenders hit the campaign trail Thursday, with Mr. Obama rallying supporters in Florida. Mr. Romney addressed supporters in North Carolina before plans to crisscross the battleground states of Virginia and Ohio on Friday. Those four states hold a combined 75 of the 270 electoral votes needed to win the November 6 election.

The Des Moines Register newspaper in Iowa, another crucial swing state in the November election, published an interview Wednesday with Mr. Romney in which he declared he would not pursue any legislation aimed at restricting or outlawing abortion if he were elected.

The Obama campaign accused Mr. Romney of contradicting his previous anti-abortion stance. But speaking to reporters in Ohio Wednesday, Mr. Romney insisted that he is a “pro-life candidate” and would be “a pro-life president.”