Sweet column: Clinton apology to Obama. Obama "looking forward" to her advising Obama White House.

MAQUOKETA, Iowa — Sen. Hillary Rodham Clinton (D-N.Y.) personally apologized to Sen. Barack Obama (D-Ill.) on Thursday for comments one of her presidential campaign co-chairmen made about Obama’s youthful drug experimentation, as Obama warned her not to engage in negative tactics.

This extraordinary exchange, lasting about 10 minutes, according to Obama chief strategist David Axelrod, came on the tarmac at Reagan National Airport, where charter planes for the senators were waiting to fly them to Iowa for the final Democratic debate before the Jan. 3 caucus.

The debate, sponsored by the Des Moines Register, turned out to be fairly sedate, and in contrast to the last two times the Democrats were on a stage together, fairly free of intense exchanges, with the intent to end on a high note with only days left before the leadoff presidential vote.

Clinton’s bid to do this was marred as a co-chairman, Bill Shaheen, a longtime New Hampshire political activist whose wife, Jeanne, is a former governor and current U.S. Senate candidate, resigned from the campaign Thursday, a day after he told the Washington Post that Obama’s past drug use may be used against him in a general election campaign.

In a statement issued after the debate, Shaheen said, “Senator Clinton has been running a positive campaign focused on the issues that matter to America’s families.

“… 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.”

Shaheen had said Republicans have ammunition to strike from admissions Obama made about using cocaine and pot in his memoir, Dreams From My Father.

Shaheen’s comments came as the issue of electability is becoming more of a factor, with the campaigns of Clinton, Obama and former Sen. John Edwards (D-N.C.) sparring over who is best positioned to win a general election.

Though the Clinton campaign said Wednesday that Shaheen was acting on his own, Clinton sought out Obama to make an apology while still in Washington, said spokesman Jay Carson. Carson said she told Obama, “I think you want to hear this from me.”

Axelrod said the apology was a “nice gesture,” and he “accepts” her word that Shaheen was working solo. Obama told Clinton she needed to “send the message from the top down” that “negative campaigning is not part of the process,” Axelrod said.

Asked about Obama’s past with drugs not covered in his book, Axelrod said Obama, 46, never sold drugs and that his drug use ended in his 20s.

Obama campaign manager David Plouffe used the Shaheen incident in an appeal for $25 donations, saying Shaheen was “smearing Barack.”

At the debate, Clinton daughter Chelsea was in the audience (Clinton’s mother, Dorothy Rodham, is featured in a new commercial), and Obama was asked how he could argue he is breaking from the past when he has former President Bill Clinton advisers on his team.

Laughing, Hillary Clinton said, “I want to hear that.”

Shot back Obama, “Well, Hillary, I’m looking forward to you advising me, as well.”

Clinton noted that all of the Democrats are calling for change. Not calling out Obama or Edwards by name — but referring to them and not Sen. Chris Dodd (D-Conn.), Sen. Joe Biden (D-Del.) and New Mexico Gov. Bill Richardson — she said that “some believe you get change by demanding it, some believe you get it by hoping for it. I believe you get it by working hard for change.”

After the debate in a Des Moines suburb, Obama started on an Iowa tour running through Tuesday, with a stop Thursday with Oprah Winfrey pal Gayle King.