Bill Shaheen, one of New Hampshire's most powerful Democrats, resigned yesterday as co-chairman of Hillary Clinton's national and state campaigns after invoking Barack Obama's past use of illegal drugs into the rhetoric of an increasingly heated battle for the party's presidential nomination.

Shaheen had apologized Wednesday night for telling a reporter that if Obama is nominated, Republicans will use against him his own admission that he "got into drinking" and "experimented with drugs" as a youth.

Shaheen, a Madbury resident who practices law in Dover, is the husband of former Gov. Jeanne Shaheen, now a candidate for the U.S. Senate. Yesterday afternoon, he said in a statement, "I made a mistake and in light of what happened, I have made the personal decision that I will step down."

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Clinton early yesterday personally apologized to Obama for Shaheen's remarks. The two candidates reportedly met at Ronald Reagan Washington National Airport before appearing together in a debate in first-caucus state Iowa.

SHAHEEN (THE ASSOCIATED PRESS)

Shaheen said, "I would like to reiterate that I deeply regret my comments yesterday and say again that they were in no way authorized by Senator Clinton or the Clinton campaign. Senator Clinton has been running a positive campaign focused on the issues that matter to America's families.

"She is the best qualified to be the next President of the United States because she can lead starting on day one," Shaheen continued. "I made a mistake and in light of what happened, I have made the personal decision that I will step down as the Co-Chair of the Hillary for President campaign. This election is too important and we must all get back to electing the best qualified candidate who has the record of making change happen in this country. That candidate is Hillary Clinton."

Shaheen was named co-chairman of the state and national Clinton campaigns, an unpaid post, in March. He told the Washington Post on Wednesday that if Obama is the Democratic nominee, "The Republicans are not going to give up without a fight, and one of the things they're certainly going to jump on is his drug use."

Shaheen said Obama has invited further questions by being so open on the topic.

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

Obama wrote about his teenage drug use in his memoir, "Dreams from My Father," saying that at one point in his life, he was headed to be a "junkie" and "pothead" -- "the final fatal role of the young would-be black man." He said he smoked marijuana and drank alcohol, and would snort cocaine at times.

Obama told Manchester Central High School students last month, "I made some bad decisions that I've actually written about. You know, got into drinking. I experimented with drugs. There was a whole stretch of time that I didn't really apply myself a lot. It wasn't until I got out of high school and went to college that I started realizing, 'Man, I wasted a lot of time.'"

Last night, after Shaheen's resignation, Obama spokesman Reid Cherlin said in a statement that "the politics of false attacks and innuendo are being rejected by voters from New Hampshire and around the country. Barack Obama is focusing his campaign on the positive change he is going to bring to America as President."

Obama campaign manager David Plouffe was far more direct in a fund-raising e-mail to supporters.

"The only way to stop these kinds of tired, desperate attacks is to demonstrate very clearly that they have a real cost to Senator Clinton's campaign," he wrote. "Make no mistake -- this kind of attack is becoming a pattern as Clinton's support declines."

Top state Obama supporters, former state Health and Human Services Commissioner Ned Helms and state Sen. Martha Fuller Clark, also criticized the Clinton campaign and Shaheen at a morning news conference.

"The political textbook says when you're not doing well in the polls, you attack your opponent," Clark said.

Polling has shown Clinton's once-huge lead diminish to single digits in the last weeks of the New Hampshire presidential primary campaign. One poll this week showed Obama slightly ahead.

Helms said a disturbing pattern has emerged, first about Obama's presidential aspirations as a kindergartner, then e-mails on the Muslim religion, and now on past drug use.

"When you see a pattern of people making statements, then a follow-up statement that it wasn't authorized, it doesn't take a genius to see there's a thread going on here," Helms said.

Helms and Shaheen are contemporaries in New Hampshire Democratic politics. Both were appointees of the late Gov. Hugh Gallen -- Helms as state health commissioner and Shaheen as judge of the Durham District Court.

Helms said after Shaheen's resignation that he had spoken earlier yesterday with former Gallen chief of staff Dayton Duncan, another veteran campaign strategist who is now on the Obama steering committee.

"Dayton said to me that if one of us had said this, both of us would have felt strongly that the other person should step down," Helms said, "because this is not what our campaign is about."

He said that he was attracted to Obama because "right from the very beginning," the campaign set a positive tone and "because it was about the future."

Helms said that when Clinton said recently in Iowa that the "fun part" of the campaign was to criticize other candidates, "that sends a broad message throughout the campaign.

"I can't speak to Billy's motivation," Helms said, "but each campaign sets a tone and this campaign has set a tone right from the very beginning that we're going to talk about the future of this country in a positive way."

State Rep. Jim Splaine, D-Portsmouth, a Clinton supporter, posting on the Democratic Web site BlueHampshire.com, defended Shaheen as "a class act" who "made a serious error." He said that "one bad day should not forever supplant the many good things (Shaheen) has done for New Hampshire Democrats."

It remains to be seen how seriously the Shaheen episode "plays into the (Clinton) pattern of negative, dirty campaigning and a campaign that says one thing and does another thing and then does another thing," said Andrew Smith, an associate professor of political science at the University of New Hampshire. "The fact that he resigned so quickly may stem the bleeding a bit, but they've still got some work to do" to offset the negative perception.