WASHINGTON (CNN) -- One of Sen. Hillary Clinton's top advisers is stepping down after saying Sen. Barack Obama's admission of past drug use would hurt his chances in a general election matchup.
Sen. Barack Obama says he doesn't think his admission of past drug use will hurt his presidential campaign.
"I would like to reiterate that I deeply regret my comments yesterday and say again that they were in no way authorized by Sen. Clinton or the Clinton campaign," Bill Shaheen, co-chairman of the Clinton campaign in New Hampshire, said in a statement announcing his decision.
Earlier Thursday, Clinton personally apologized to rival Obama for Shaheen's remarks.
Obama accepted her apology, according to David Axelrod, the top political strategist for the Obama campaign.
"[Obama] said it's important for campaigns to send a signal from the top. If they say that negative campaigning is fun, they're sending a signal. Or that can say this kind of thing isn't tolerated," Axelrod said.
Obama earlier brushed aside the adviser's remarks, saying they were probably the result of the latest poll numbers, which show the two candidates tied in the first primary state, New Hampshire.
"I just think people are feeling a little worried about the polls," the Illinois senator said Thursday.
Shaheen told a Washington Post reporter Wednesday that Republicans would exploit Obama's admission of past drug use should he win the Democratic Party's presidential nomination and might even suggest Obama once dealt drugs.
"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. "It'll be, 'When was the last time? Did you ever give drugs to anyone? Did you sell them to anyone?' There are so many openings for Republican dirty tricks. It's hard to overcome."
Shaheen, husband of former New Hampshire Gov. Jeanne Shaheen, apologized for his comments and said in a statement late Wednesday "they were not authorized by the campaign in any way."
Obama said he did not think it was Shaheen's intent to plant such a rumor. The candidate said he is not worried what Shaheen said may damage his campaign "since it's not true."
"I don't think the American people are concerned about what I did when I was a teenager. They're concerned about what I'm going to do as a presidential candidate," he said.
Clinton on Thursday said she did not approve of Shaheen's comments. "I don't in any way condone that, and it was certainly not authorized or approved at all. It was inappropriate, and he's apologized," the senator from New York said.
Obama spokesman David Plouffe called the remarks desperate, saying the "campaign is recycling old news that Barack Obama has been candid about in a book he wrote years ago, and he's talked about the lessons he's learned from these mistakes with young people all across the country."
Obama wrote in his 1995 book "Dreams of My Father" that he was once headed in the direction of a "junkie" and a "pothead."
Referring to his emotional struggles as a young man, Obama wrote, "Pot had helped, and booze; maybe a little blow when you could afford it. Not smack, though."
Obama has not shied away from bringing up his past use of drugs while on the campaign trail. He spoke candidly about his past experimentation at an event in New Hampshire last month, acknowledging that he did inhale and that "it's not something I'm proud of."
And in a statement that proved popular with the crowd, Obama said he never understood then-presidential candidate Bill Clinton's contention in 1992 that he did not inhale marijuana.
"I never understood that line," he said. "The point was to inhale. That was the point."
The latest back-and-forth between the two Democratic campaigns on the issue of Obama's electability came as a new CNN/WMUR poll showed the two candidates statistically in a dead heat in New Hampshire.
Clinton stands at 31 percent, with Obama 1 percentage point behind, well within the poll's margin of error of plus or minus 5 percentage points. E-mail to a friend
CNN's Alex Mooney and Ted Barrett contributed to this report.