U.S. Vice President Joe Biden has called last month's fatal attack against the U.S. ambassador to Libya “a tragedy,” promising that whatever “mistakes” were made one month ago “will not be made again.”

In a nationally televised debate at a small college in Kentucky, the Republican presidential challenger's running mate, Representative Paul Ryan, slammed the Obama administration for not providing enough security in Benghazi and for taking “two weeks to acknowledge that this was a terrorist attack.”

“Our ambassador in Paris has a marine detachment guarding him. Shouldn't we have a marine detachment guarding our ambassador in Benghazi, a place where we knew that there was an Al-Qaida cell with arms?”

The vice president argued that President Barack Obama was not told that the consulate wanted more security.

On Iran, Ryan said the Islamic Republic has become “brazen” because the Obama administration has “no credibility” on the issue of Iran's nuclear program. Biden countered this claim, asserting that the U.S. has placed “the most crippling sanctions in the history of sanctions” on Iran.

The debate late Thursday 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 mistakes in his public statements.

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.

The presidential contenders were back on 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.