The race looks poised to swing his way after a series of votes, beginning tomorrow with caucuses in Washington state, Nebraska and the Virgin Islands and a primary in Louisiana.

The Clinton campaign team anticipates that she will lose her lead in the delegate count this month but is banking on her regaining the lead in the mega-states of Texas and Ohio on March 4 and Pennsylvania on April 22.

It will be psychologically and symbolically important for Obama to take the lead after trailing for so long, albeit not by much. He is already ahead of his rival in fundraising.

The Clinton campaign is expected to change its tack following the emergence of John McCain as the likely Republican adversary.

The Clinton camp is expected to argue that she is better placed to deal with McCain on security and economics. Her team is also expected to bring up a testy exchange of letters between McCain and Obama in 2006.

McCain accused Obama of backtracking on a promise to support him over a bill to restrict lobbying. He accused Obama of "self-interested partisan posturing". Obama responded to say he was "puzzled" by McCain's outburst.

"It was sad to watch," said a Clinton campaign source. "He has no idea how to deal with McCain."

The two are seeking to win 2,025 delegates, the magic number that would give them a majority of delegates at the party convention in August where the Democratic nominee for the November presidential election will be chosen.

The final tally of delegates has still not been allocated after Super Tuesday. The 2008 Democratic Convention Watch website today gave Clinton a total of 862 elected delegates to Obama's 883.

But when super-delegates - the 700-plus members of Congress, governors and others who automatically have a vote at the conventions - are taken into account, she has 1,065 and Obama 996.

Clinton's team expects Obama to take a bigger share at some point this month. After this weekend, Virginia, Maryland and Washington DC vote on Tuesday and Wisconsin and Hawaii on February 19.

There is growing concern in the Democratic party that the two could exhaust one another, deplete funds and damage one another with day-to-day criticism at a time when the Republicans have more or less settled on John McCain.

But the Republicans are by no means united behind their candidate, and president George Bush intervened in the race for the first time today to call on his divided party to back the nominee, though without naming McCain.

Many conservative Republicans are openly hostile towards McCain because of his moderate stance on immigration and his willingness to work with Democrats in the Senate.

While McCain will distance himself from the unpopular president in the run-up to the November election, Bush's endorsement could help overcome the hostility of the Republican base towards him. He needs those Republican activists to work for him in the November election.