WASHINGTON : Democratic chief Howard Dean Friday warned Barack Obama and Hillary Clinton not to rip the party apart with their bitter White House battle and tried to head off a divisive convention fight.

Dean spoke out as veteran senator and Obama supporter Patrick Leahy made the most explicit call yet from a party luminary for Clinton to drop out of the race, and permit the Illinois lawmaker to take on Republican John McCain.

Dean, chairman of the Democratic National committee, told CBS that "the candidates have got to understand that they have an obligation to our country to unify."

"Somebody's going to lose this race with 49.8 percent of the vote, and that person has got to pull their supporters in behind the nominee.

"That's our obligation. Because in the end this is not about Hillary Clinton and Barack Obama, it's about our country. We're not going to have four more years of George W. Bush, which is essentially what McCain is offering us."

Dean also said he favored a solution which would see party graybeards, or superdelegates effectively crown the nominee long before the Democratic convention in August in Denver.

"I think there's 800 of them and 450 of them have already said who they're for. I'd like the other 350 to say who they're on between now and the 1st of July so we don't have to take this into the convention."

Neither Clinton nor Obama can reach the magic number of 2,025 delegates necessary to wrap up the nomination, so the choice of superdelegates will be decisive.

Leahy's comments on his home state Vermont Public Radio, are likely to generate seething anger in the Clinton camp.

"There is no way that Senator Clinton is going to win enough delegates to get the nomination," he said.

"She ought to withdraw and she ought to be backing Senator Obama. Now, obviously that's a decision that only she can make."

"John McCain, who has been making one gaffe after another, is getting a free ride on it because Senator Obama and Senator Clinton have to fight with each other."

Leahy and Dean were giving voice to increasing current concern among leading Democrats that the internecine warfare could scupper a golden chance to win the White House from a ragged Republican party, as the US economy staggers.

But the DNC chairman and unsuccessful 2004 presidential candidate, has little power to force either hopeful out of the race, in which Obama currently leads in both pledged delegates and the popular vote.

On Thursday, Clinton was asked while campaigning in North Carolina what she would say to Democrats who might consider voting for McCain, if their preferred candidate loses in the noxious party tussle.

"Please think through this decision," Clinton said. "It is not a wise decision for yourself or your country."

"I intend to do everything I can to make sure we have a unified Democratic party ... when this contest is over and we have a nominee, we're going to close ranks, we're going to be united."

Obama, speaking to ABC News, admitted that whichever camp loses the race is likely to suffer "bruised feelings."

"It's tough ... we have been campaigning now for a long time. We have got very ardent supporters on both sides.

"I don't think we are hurt, long-term. I think short term, there is going to be work to do for the nominee to bring the party back together again," Obama said.

"We are going to have to come together and remind ourselves that there is a heck of a lot bigger difference between either Senator Clinton or myself, and John McCain."

Obama was to spend the weekend barnstorming in Pennsylvania, which votes April 22, while Clinton's was heading for Indiana, which votes May 6.

As the Democrats squabbled, McCain meanwhile cranked up the pace of his general election effort, debuting a campaign ad stressing his military heroism and incarceration during the Vietnam war.

"The American president Americans have been waiting for," says the narrator in the ad, which includes film of navy pilot McCain being asked for his military identification number "624787" as he lies wounded after being shot down in 1967.

The advertisement was the latest sign that the 71-year-old Arizona senator, who strongly backs the Iraq war, wants to make November's general election about who can best keep Americans safe.

- AFP /ls