Clinton was asked about the official's comments aboutas she campaigned in Iowa, where the controversy has become an issue less than three weeks before the state's leadoff caucuses.
"As soon as I found out that one of my supporters and co-chairs in New Hampshire made a statement, asked a series of questions, I made it clear it was not authorized, it was in no way condoned, I didn't know about it and he stepped down," Clinton said.
A day earlier, Bill Shaheen, a national co-chairman for Clinton and a prominent New Hampshire political figure, had resigned. He and the Clinton campaign had been criticized after he suggested Obama's admitted use of drugs as a teenager could be used against the Illinois senator if he became the Democratic presidential nominee.
Clinton, speaking during a taping of Iowa Public Television's "Iowa Press" program and during a meeting with reporters afterward, sought distance from that comment.
"I made it very clear as soon as I heard about it that I not only disapproved, it did not reflect the campaign I am running," said Clinton. "I did personally apologize, the gentleman in question has stepped down from the leadership role in my campaign."
Asked if the issue of Obama's drug use should be an issue, Clinton said, "Not in my campaign."
"There are a lot of differences between us, and those are the contrasts that should be drawn," said Clinton. "I'm running a campaign about who I am, what I've done and that's what I'm going to stay focused on.'
She also rejected recent comments from her campaign about Obama saying he hadn't sought the presidency for long - after writing and talking about such an ambition throughout his life.
"That was silly, and I told my campaign it was silly," Clinton said. "My whole point has been there are legitimate differences."
Clinton made it clear that she wouldn't hesitate to point out difference with rivals such as Obama.
"There has to be an exchange of views, there has to be a legitimate discussion of the contrasts among us," said Clinton. "I reject completely the kind of line-crossing that I've stood up against in my campaign consistently. As soon as we find out something happened that we don't authorize, we don't condone, we have no part of, we ask people to please not be a part of our campaign."
While Clinton has built a lead in national polling on the Democratic candidates, she is in a tight race with Obama and former Sen.in Iowa.
"I always thought this would get close," said Clinton, who noted she doesn't have the advantage of coming from a neighboring state like Obama or a long history in the state like Edwards.
"This is what happens in a contested election, and that's the best of our democracy," said Clinton.
She dismissed criticism of her as a divisive figure.
"I've been tested, I've been vetted, I've been in the political arena in our country for 16 years, there are no surprises," said Clinton.
As the race has tightened, some have suggested a shake-up is coming in her campaign, a suggestion she rejected.
"I saw some of those articles, and I called my campaign and said, "Do you know anything about this? Are you keeping something from me?" said Clinton, who added that she has confidence in her campaign staff.
She said the race is fluid by nature.
"Politics now is a 24/7 cycle. You go up, you go down," said Clinton. "I think that's all part of a vigorous, dynamic election cycle."
During her swing, Clinton collected the endorsement of Iowa Rep. Leonard Boswell, and he joined her session with reporters.
"As a family, we endorse Hillary because we want to win," said Boswell. "I'm not against anybody on the ticket, I want somebody who can win."
"He knows I can win," said Clinton. "We have to win and that's the bottom line."