DENVER - The man who will question President Obama and Republican nominee Mitt Romney on Wednesday once compared moderating a presidential debate to "walking down the blade of a knife."

Jim Lehrer, a longtime anchor of "PBS NewsHour," also pledged he'd never moderate another debate after his 11th turn at the helm in 2008. But he changed his mind this year when the Commission on Presidential Debates proposed an expanded format that will include six 15-minute segments.

The new format, Lehrer said recently, "has the possibility to open up the debates for a more spontaneous and deeper exchange of positions and ideas."

At 78, Lehrer is the oldest of the moderators chosen this election season, but just barely. The commission chose Bob Schieffer, 75, of "CBS News," to moderate the final presidential debate on Oct. 22 in Boca Raton, Fla. CNN's Candy Crowley, 63, will ask the questions at the second debate in Hempstead, N.Y., on Oct. 16.

Lehrer is considered the most experienced of the trio when it comes to presidential debates and is often called "the dean of moderators."

His first turn in the moderator's chair was 24 years ago, during the 1988 presidential contest between then-Vice President George H.W. Bush and the Democratic nominee, Massachusetts Gov. Michael Dukakis.

Lehrer described the experience in his book, "Tension City: Inside the Presidential Debates," writing that he refused the last-minute urging of "ABC News" anchor Peter Jennings, a co-moderator, to throw out the rules and call on Bush and Dukakis to take each other on directly.

Lehrer blocked the move, calling it "dishonorable" because it would disregard the format set by the debate commission.

Media experts say Lehrer's predictability and emphasis on substance over flash is a main reason the debate commission asks him to come back and moderate every election cycle.

"I think he's going to ask substantive questions but not gotcha questions," Stacey Woelfel, an associate professor at the Missouri School of Journalism, told The Washington Examiner. "His questions don't set up the candidate for one-line zinger sort of answers, but they will have to give substantive answers."