The first indication of a change of tack came from Hillary Clinton’s camp, which stepped up its fundraising this weekend in case the struggle against Barack Obama has to be taken beyond Super Tuesday. Her advisers told the New York Times they aimed to raise $10m by the end of this month, on top of their existing war-chest of $20m, for post-Super Tuesday spending.

More than 20 states vote on February 5, including California, New York and Illinois, which between them command hundreds of delegates to the parties’ official nomination conferences. Perceived wisdom has been that the nominee of both main parties will be all but crowned on the night of Super Tuesday, but with the Clinton-Obama race tight, and the Republican pack potentially split four ways between John McCain, Mike Huckabee, Mitt Romney and Rudy Giuliani, it is possible that a decisive result will only emerge later.

The campaign between the two Democratic frontrunners has already grown tetchy, with Clinton on NBC’s Meet the Press programme on Sunday accusing the Obama campaign of ‘deliberately distorting’ her comments about Martin Luther King.

She said they misinterpreted her words to give the impression that she was playing down King’s role in the civil rights movement. Her remarks that it took President Lyndon Johnson to effect civil rights legislation have been seized on by black leaders in South Carolina, where Democrats vote on January 26. The unions have also been squabbling over how the next Democratic election should be held in Nevada on January 19.

Obama has been endorsed by the Culinary Workers’ Union, and for the first time caucuses will be held in nine casinos along the Las Vegas strip in a bid to increase the vote among service workers. But another union, the Nevada State Education Association, which has yet to endorse a candidate, is taking legal action to stop the caucuses, arguing that they will give Obama an unfair advantage.

A CNN/Opinion Research poll at the weekend suggested that whichever Democratic candidate emerged victorious would hold a substantial lead over three of the four Republican contestants — Romney, Huckabee or Giuliani.

Only McCain performed competitively in the poll — respondents put him on 48%, compared with 49% for Obama and 50% for Clinton.