Hillary Clinton apologised to Barack Obama last night after one of her advisers attempted to highlight the past drug use of her main rival for the Democratic presidential nomination.

The remarks by Bill Shaheen, the chairman of Mrs Clinton’s team in New Hampshire, added to a sense of desperation inside the former First Lady’s once formidable campaign. Following the furore Mr Shaheen was forced to resign from Mrs Clinton’s team.

“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,” Mr Shaheen said.

His offensive comments about Mr Obama were the latest in a series of attacks that have backfired as Mrs Clinton’s campaign struggles to halt his rise in the polls.

Her aura of inevitability gone after a damaging month, Mrs Clinton apologised to Mr Obama on an airport tarmac as they headed to Iowa for the final televised debate before the state’s January 3 caucuses. The debate was cordial and uneventful.

Polls released this week show that Mrs Clinton’s enormous lead in the second nominating state of New Hampshire has gone. Only last month she had nearly a 20-point advantage over Mr Obama in the Granite State, which was seen as a firewall for her if she failed to win Iowa. Now the two are tied.

In South Carolina, the third state to vote and where Mrs Clinton also enjoyed a large lead recently, Mr Obama has again wiped out her advantage.

Mr Shaheen had raised questions in an interview about Mr Obama’s admission in a memoir that he had used drugs in his youth. Mr Obama wrote in Dreams from My Father, published in 1995, that he smoked marijuana and took cocaine when he was a teenager.

Mr Shaheen argued that the admission would leave Mr Obama open to Republican attacks if he won the Democratic nomination. “It’ll be: ‘When was the last time? Did you ever give drugs to anyone? Did you sell them to anyone’?” Mr Shaheen had said.

“There are so many openings for Republican dirty tricks. It’s hard to overcome.” The Clinton campaign immediately distanced itself from the remarks, prompting Mr Shaheen to quit.

However, the Clinton campaign had sent reporters a memo implying that Mr Obama lied when he said that he only recently thought about running for the presidency. It cited an essay he wrote in kindergarten entitled “I want to be President”.

After Mr Shaheen’s comments, David Plouffe, Mr Obama’s campaign manager, said: “Hillary Clinton has . . . moved from Barack Obama’s kindergarten years to his teenage years in an increasingly desperate effort to slow her slide in the polls.”

There are reports of increasing dissent inside the Clinton camp. Bill Clinton believes that the campaign should take on Mr Obama more aggressively. But such a strategy carries high risks, especially in Iowa, where negative campaigning has backfired in past elections.

Mrs Clinton’s troubles began with a poor debating performance on October 30. Since then her team has been exposed for planting questions at campaign events. On Monday a second Iowa volunteer resigned after forwarding a hoax e-mail claiming that Mr Obama was a dangerous Muslim.

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