"I think it was clear that I got in too late last time," the retired general and former NATO commander told The Associated Press in an interview.
Clark announced his bid for the Democratic presidential nomination in September 2003, just four months before the first votes were cast. He dropped out of the race the following February, with his only victory in 14 caucuses and primaries coming in Oklahoma.
Clark is considering running again, but he said the late start was one of the mistakes he learned from in his last attempt.
"(There was) an inability to create a staff in a timely fashion," Clark said. "I didn't have a campaign manager until the end of November. I had no money. I had no strategy when I started. It was my only faith-based initiative.... It's one of several mistakes that if I were to run that I would hope I wouldn't repeat."
Clark said one factor that has changed is the number of candidates who have announced their intentions or have formed exploratory campaigns for president this early.
"There's no doubt it's moved earlier," Clark said. "Some people have never stopped running.... It's just a factor that has to be considered."
Clark says he believes his military background has special resonance during this campaign because of the national security issues that face the country, including the war in Iraq.
"I think that it's more clear than ever before that the country is an era of profound national security challenges," Clark said.
Clark said Iraq will be a focus of the 2008 campaign. He disagreed with suggestions by some members of Congress that more U.S. troops should be sent to help stabilize Iraq. Neither would he begin reducing U.S. forces in Iraq within the next six months, as others have suggested.
"It's not a matter of fiddling with troop levels," Clark said. "It's a matter of politics inside Iraq and diplomacy in the region. ... You can lose what's going on militarily inside Iraq, but you can't win it militarily, either by putting more forces in or by pulling them out."
Clark was a popular surrogate on the campaign trail during the midterm elections and has visited New Hampshire in the past month.
He said he hasn't set a deadline for making a decision on the presidential race.
"I don't deal with timelines," Clark said. "For me, it's talking to people and working separate related issues."