JOHNSTON, Iowa — The resignation of the Clinton campaign’s New Hampshire co-chairman over his speculation that Senator Obama was a drug pusher as well as a user is overshadowing the final pre-caucus Democratic debate here and threatens to throw both Democratic presidential front-runners off track.

Mrs. Clinton has disavowed the comments by her New Hampshire co-chairman, Bill Shaheen, and the Clinton campaign and Mr. Shaheen insist the remarks were unauthorized. But the flap — which, by virtue of Mr. Shaheen’s resignation yesterday is today entering its third day in the news cycle — has forced the Obama campaign to confront the Illinois senator’s teenage transgressions in a much harsher light than it has thus far.

The Clinton campaign announced the departure of Mr. Shaheen minutes after the conclusion of a tepid debate between the Democratic contenders in Iowa. The resignation came less than a day after he 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.

“It’ll be, ‘When was the last time? Did you ever give drugs to anyone? Did you ever sell them to anyone?” Mr. Shaheen had said. “There are so many openings for Republican dirty tricks. It’s hard to overcome.”

The issue clearly could hurt Mrs. Clinton, who has faced repeated accusations of negative and even dirty campaigning. And her top surrogates yesterday were peppered with more questions about Mr. Shaheen than about her debate performance.

But it could also derail Mr. Obama in the way Mr. Shaheen seemed to envision when he said that Mr. Obama’s candor about his drug use as a teenager — which the senator admitted in his memoir and has discussed on the campaign trail — would open the door for more probing questions in a general election campaign. A senior adviser to the Obama campaign, David Axelrod, had to answer 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 Mrs. Clinton’s 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 campaign even tried to swing the dispute in its favor by sending a fund-raising letter to supporters based on Mrs. Clinton’s “negative” campaigning.

The spat between Mrs. Clinton and Mr. Obama could 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 Edwards campaign, Joseph Trippi, said it would damage 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 demurred.

But in a separate appearance on MSNBC he did suggest the Clinton campaign was intentionally exploiting the drug use issue. After Clinton strategist Mark Penn casually mentioned Mr. Obama’s “cocaine use” while defending the campaign, Mr. Trippi jumped in. “He said it again. He said it again. Unbelievable,” Mr. Trippi said, as he stood next to Mr. Penn on camera. “He said cocaine again.”

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,” said one veteran Republican strategist who didn’t want to comment publicly on the issue of the senator’s past drug issue.

The bickering over Mr. Shaheen was 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 Wednesday.

Facing mostly substantive policy questions, the candidates avoided direct criticism of each other. 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 interjected as she continued to laugh. Mr. Obama retorted: “Well, Hillary, I hope you’ll be advising me as well.”

The candidates were unanimous in support of raising taxes on the wealthy to pay for domestic programs, such as universal health care. “I want to restore the tax rates we had in the 90s. That means raising taxes on corporations and wealthy individuals,” Mrs. Clinton said. They also addressed farm subsidies, a key issue in Iowa.

Citing New York City as an example, Mr. Obama voiced support for capping large subsidies as well as leveling the playing field for family farmers. “We’ve got folks in Manhattan who are getting farm subsides, Fortune 500 companies who are getting farm subsidies, and as a consequence, family farms are getting squeezed out,” he said.

The debate was the final meeting of the Democrats in Iowa before the caucuses on January 3. Mr. Obama now leads in several polls, but with Mrs. Clinton and Mr. Edwards bunched close behind, the race is considered a virtual three-way tie.