With polls narrowing less than five weeks before Election Day, Obama and Romney launched a new phase in a bitter race dominated so far by negative advertising as both camps try to frame the election to their advantage.

Whether it matters is itself a topic of debate. According to an analysis by Gallup, televised debates have affected the outcome of only two elections in the past half century -- Nixon-Kennedy in 1960 and Bush-Gore in 2000.

Both candidates had their wives in the audience at the University of Denver in Colorado for the debate taking place on the 20th wedding anniversary of the president and first lady Michelle Obama.

Obama opened the debate by promising his wife they wouldn't be celebrating their anniversary next year in front of 40 million people, and Romney joked that Obama found the most romantic place possible for the anniversary.

Analysts say Obama needed a presidential performance rather than fireworks or haymakers in order to maintain and build on a narrow edge in polls that indicate a very close election on November 6.

Romney, who has been unable to catch the president in most of the polls to date, sought to generate enthusiasm for a change in the White House as the nation wrestles with seemingly chronic economic problems such as mounting federal deficits and debt.

Lehrer, moderating his 12th presidential debate, planned to break up the debate into 15-minute segments focusing on different aspects of the economy and other domestic issues. However, the exchanges by the candidates scrambled the format, with the opening discussion on taxes lasting more than 20 minutes.

The two candidates shook hands and shared a laugh after being introduced by Lehrer as the audience applauded before being asked to remain silent for the remainder of the debate. At one point, a loud bang off-stage seemed to surprise Romney in mid-sentence, and Obama looked behind him to try to see what happened.

The other presidential debates will occur on October 16 in New York and October 22 in Florida. Vice President Joe Biden and Rep. Paul Ryan of Wisconsin, Romney's running mate, will debate on October 11 in Kentucky.