JOHNSTON, Iowa — The resignation of a top New Hampshire adviser to Senator Clinton over his unauthorized criticism of Senator Obama’s past drug use is overshadowing the final Democratic debate here and threatens to throw both presidential front-runners off track.

The Clinton campaign announced the departure of its New Hampshire co-chairman, Billy Shaheen, minutes after the conclusion of a tepid debate between the Democratic contenders in Iowa. The resignation came less than a day after Mr. Shaheen publicly raised the specter of Mr. Obama’s acknowledged use of cocaine and marijuana as a campaign issue, telling a Washington Post reporter that Republicans could use it to torpedo the Illinois senator’s campaign if he won the Democratic nomination.

The issue could hurt Mrs. Clinton, whose advisers today were peppered with questions about Mr. Shaheen’s comments instead of the relatively solid performance she gave at the debate.

But it could also derail Mr. Obama in the way Mr. Shaheen seemed to envision when he said that his candor about his drug use as a teenager would open the door for more probing questions in a general election campaign, such as whether he had ever given or sold drugs to others. A senior adviser to the Obama campaign, David Axelrod, faced precisely those queries from reporters in a scrum after the debate. He said emphatically that Mr. Obama had not sold or given out drugs.

Before the debate, Mrs. Clinton personally apologized to Mr. Obama when they ran into each other at a Washington airport on the way to Iowa. Mr. Shaheen had apologized for his remarks last night but in a statement today he said he made a “personal decision” to step down.

Mr. Axelrod said that Mr. Obama accepted her apology and had not explicitly called for Mr. Shaheen to resign. Obama advisers had initially responded to the remarks by saying they smacked of “desperation.”

The spat between Mrs. Clinton and Mr. Obama appears to help John Edwards, the former North Carolina senator who has been running a close third in Iowa polls. Mr. Edwards’s wife, Elizabeth, wanted nothing to do with the dispute — “You are absolutely not going to get a comment out of me on that,” she told The New York Sun after the debate. But a senior adviser to the campaign, Joseph Trippi, said it would be damaging to the Clinton campaign and “underscored some of the meanness” coming from her shop since she began slipping in the polls. “She keeps emerging as the sort of status quo political campaign that people are sick of,” he said. “It’s one self-inflicted wound after another.”

As for whether it would hurt Mr. Obama, Mr. Trippi said: “Who knows?”

Republicans were privately gleeful. “It seems like an act of desperation from the Clinton campaign. At the same time, it’s a storyline that I can’t imagine is very helpful to Senator Obama,” one veteran GOP strategist said, who didn’t want to comment publicly on the issue of the senator’s past drug issue.

The bickering over Mr. Shaheen was decidedly more combative than anything the candidates said during the debate, which was held by the Des Moines Register and largely mirrored the polite affair the Republicans held here yesterday.

Facing mostly substantive policy questions, the candidates avoided direct criticism. Mrs. Clinton came the closest to engaging her rivals, delivering a veiled jab at Mr. Edwards’s confrontational tone during the campaign and Mr. Obama’s message of a “politics of hope.” “Everybody on this stage has an idea of how to get change,” she said. “Some believe you get change by demanding it. Some believe you get it by hoping for it. I believe you get change by working hard for it.”

The former first lady was also involved in one of more awkward moments of the debate. She began laughing loudly when Mr. Obama was asked how he could deliver a change in foreign policy considering his top advisers were all senior aides to President Clinton. “I want to hear that,” Mrs. Clinton said as she continued to laugh. Mr. Obama retorted: “Well, Hillary, I hope you’ll be advising me as well.”

The debate was the final meeting of the Democrats in Iowa before the caucuses on January 3rd. Mr. Obama now leads in several polls, but the race is considered too close to call.