· Rival ahead for first time in New Hampshire · Campaign team planning to step up attacks on rival

Hillary Clinton's campaign for the White House suffered a serious setback yesterday when her main Democratic rival, Barack Obama, took a poll lead for the first time in New Hampshire, a key early state.

The latest ratings, part of a trend that has seen him narrow the gap over the last month, has rattled the Clinton team.

In a sign of its unease, Bill Shaheen, a co-chairman of her campaign, yesterday raised Obama's drug-taking as a teenager, even though he had admitted to it in his autobiography.

In an interview with the Washington Post website, Shaheen said the Republicans will target Obama's background. "It'll be, 'When was the last time? Did you ever give drugs to anyone? Did you sell them to anyone?"' Shaheen said.

Obama's campaign manager David Plouffe said it was "an increasingly desperate effort to slow her slide in the poll".

The poll in New Hampshire marks a dramatic change in a state where Clinton has enjoyed leads of between 10 and 20 points in more than 50 polls carried out this year. That advantage has disappeared with less than four weeks left, in what is now shaping up as a tight, unpredictable race.

A Rasmussen poll yesterday put Obama on 31% to Clinton's 28%. Two weeks ago, Clinton had a 7% lead.

Her downward trend was reflected in two other polls published yesterday. A WMUR/CNN poll put Clinton on 31%, down five points since last month, and Obama on 30%, up eight points, while a poll by Suffolk University in Boston put her on 33%, down one point, to Obama's 26%, up four, since last month.

The WMUR/CNN polling was carried out between December 6 and 10, the period that included huge media exposure for Obama as a result of campaigning in Iowa and New Hampshire alongside Oprah Winfrey. The poll put John Edwards on 16%. Edwards has concentrated on Iowa in the hope that a win there can help him close the gap in New Hampshire.

Advisers to Bill Clinton, who has been campaigning for his wife this week in Iowa, were expressing concern over the performance of the campaign team, in particular a failure to respond aggressively to attacks by Obama. Hillary Clinton's team is now planning to concentrate on Obama over the next week. It is a risky tactic - negative campaigning is often effective but can also turn off potential supporters.

The first caucus, in Iowa, is on January 3 with the first primary, in New Hampshire, on January 8. In past elections Iowa and New Hampshire have often proved decisive in determining the final outcome.

The Democratic candidates are to hold their last debate today before the Iowa caucus.

The Republicans held their debate, also in Iowa, yesterday. It was a dry, dull, safe affair. Mike Huckabee, who has enjoyed a late surge in recent weeks, was at the centre of controversy over earlier remarks aimed at Mitt Romney, a Mormon. Huckabee, a Baptist minister, said: "Don't Mormons believe that Jesus and the devil are brothers?"

Huckabee has played up religion during his campaign in an appeal to evangelical Christians, who make up about 40% of Republicans in Iowa.

Romney told NBC's Today show that "attacking someone's religion is really going too far".