Democrats came under mounting pressure Monday to close ranks behind a single candidate, as Hillary Clinton struggles to wrest the mathematical advantage from Barack Obama to win their party's nod for the White House.

The bitter presidential campaign faces weeks of harsh confrontation ahead of next month's crucial primary clash in Pennsylvania, one of 10 remaining contests to decide who will face off against Republican John McCain in November's election.

New York Senator Clinton is in an uphill battle to close the gap between her and Obama, the Illinois senator who holds a lead in the number of nominating delegates, the nationwide popular vote and the number of contests won in the campaign.

Clinton's chances of snatching a victory are receding by the day, analysts said, with respected US newspaper Politico stressing Obama would have to be "hit by a political meteor" for Clinton to win the Democratic death-match.

Clinton's own campaign reportedly has acknowledged that there is virtually no way she can finish ahead of Obama in pledged delegates.

"She will be close to him but certainly not equal to him in pledged delegates," a Clinton advisor told Politico.

Estimates show Obama leading the former first lady in pledged delegates 1,628 to 1,493, and ahead in the primary popular vote by nearly 750,000 people. The Democrats' proportional system of apportioning delegates makes it highly unlikely that Clinton could surpass Obama.

A candidate needs 2,025 delegates to secure the nomination, but with just about 500 more delegates still up for grabs it will be virtually impossible for either to win the contest outright.

That leaves the Democratic contest in the hands of 796 superdelegates, assigned by the Democratic National Committee, who can vote the way they wish at the party's national convention in Denver in August.

Independent website RealClearPolitics.com puts the superdelegate count at 248 for Clinton and 213 for Obama as of Sunday.

New Mexico Governor Bill Richardson, who threw his support behind Obama on Friday, called on the campaigns to end their brutal bickering and unite behind Obama or risk a "bloodletting" that would alienate voters in the general election.

"The Democrats (should) come together and look at who's ahead when it comes to delegates, when it comes to the popular vote, the number of states," he said Sunday on Fox news.

A possible way out of the predicament would be holding an unprecedented "mini-convention" among the superdelegates, who would decide the nomination, after the last primaries are held on June 3 in Dakota and Montana, some Democrats said.

"There would be a final opportunity for the candidates to make their arguments to these delegates, and then one transparent vote," Tennessee Governor Philip Bredesen suggested in The New York Times.

Clinton has proved resilient in recent months. She was crowned as the front-runner last year but then slipped into the role of tenacious underdog against the better-funded Obama juggernaut.

She bounced back to win strongly in Ohio and edged Obama in the popular vote in Texas, and now her aides suggests she could carry the fight all the way to August convention.

But after race-tinged controversy last week saw his poll numbers slide, Obama has re-taken the lead in the latest Gallup tracking poll, showing 48 per cent of support compared to Clinton's 45 per cent.

Clinton hopes to build a case for her candidacy with a triumph in the April 22 primary in delegate-rich Pennsylvania, where she leads in opinion polls.

Should she win big there it would reinforce her camp's argument that her winning performances in large states like California and New York leave her better positioned to carry such states in the general election, when a state's winner earns all of its electoral votes.

Pennsylvania's Democratic Governor Edward Rendell noted that if the Democratic primaries were winner-take-all instead of proportional, Clinton would be ahead by up to 70 electoral votes.

Clinton also won the states of Florida and Michigan but the delegates were not counted because they violated party rules by holding their primaries too early.

The Clinton camp has pushed hard for do-over primaries or for the original votes to count, but the states have balked, and all attempts to get their delegates to the convention have failed.