Washington: Democratic party braces for fight to the finish for White house nomination on Wednesday  as Barack Obama assailed his reinvigorated rival over taxes and, in a turnabout, her experience in foreign affairs.

Hillary Clinton, in turn, renewed her assault on Obama's resume and preparedness for the White House, a line of fire that helped produce three back-to-the-wall victories that saved her campaign from extinction on Tuesday.

Even with her hard-fought victories in the Ohio, Texas and Rhode Island primaries, Clinton barely narrowed Obama's lead in delegates to the party's national nominating convention. The party's proportional system for awarding delegates gave Obama fresh batches in each state, bolstering his claim to front-runner status.

Obama also won Vermont's primary and appeared to finish well ahead of Clinton in the Texas caucus that followed that state's primary, ensuring him an additional trove of delegates when the results are finalised over the next few days. Despite Obama's sustained delegate edge, there was an unmistakable shift in momentum as Clinton strategists crowed over their victories. Obama, in a news conference, displayed a new willingness to attack - even as his opponent entertained the idea of the two running on the same ticket in November. The Illinois senator suggested the media should be tougher on his opponent, the same case Clinton was making 48 hours earlier.

On a morning flight from San Antonio to Chicago, Obama let loose on Clinton for suggesting she was more seasoned in foreign affairs. "I have not seen any evidence that she's better equipped to handle a crisis," he told reporters just before takeoff. "If the only criteria is longevity in Washington, she's certainly not going to beat John McCain on that."

The new sniping between Obama and Clinton came as Arizona Senator McCain, the presumptive Republican nominee, sat down at the White House for lunch with President George W. Bush, followed by an endorsement in the Rose Garden.

The bitterness of the Democrats' nominating race sparked questions on whether it might undermine the eventual nominee, enabling McCain to raise money and rally Republicans.

William Galston, a senior fellow at the Brookings Institution and former Clinton White House official, suggested that the general-election defeats of Gerald Ford in 1976 and Jimmy Carter in 1980 should give Democrats pause.

History

"Modern political history suggests that when a nominating contest goes all the way to the convention, as it did for Republicans in 1976 and Democrats in 1980, the party's candidate emerges in a weakened condition," he wrote in an analysis.

For her part, Clinton lost no time renewing the accusation that Obama had overstated the significance of his opposition to the US invasion of Iraq. "Senator Obama's whole campaign is about one speech he made in 2002," the New York senator told CNN. But if Clinton's critique carried a familiar ring - it was key to ending her 11-contest losing streak - Obama's attacks were more aggressive in the aftermath of Tuesday's losses.

His campaign manager, David Plouffe, told donors in an e-mailed fundraising solicitation that Clinton "stood shoulder-to-shoulder with McCain on the worst foreign policy disaster of our generation" by voting to approve the US invasion of Iraq.

Obama's team went after Clinton for refusing to release her tax returns since her husband's presidency ended in 2001. A spokesman said the Clintons would release them "on or around April 15".

On his plane, Obama accused Clinton of exaggerating her expertise in foreign affairs. "One of the things I hope people start asking is what exactly is this foreign experience that she's claiming?" he said. "I know she talks about visiting 80 countries. It's not clear, was she negotiating treaties or agreements, or was she handling crises during this period of time? My sense is the answer is no."

Speculation

Jointticket?

Questions arose on Wednesday on whether Hillary Clinton and Barack Obama might be able to resolve their clash by running on the same ticket in the fall.

"Well, that may be where this is headed," Clinton told CBS with a laugh and broad smile. "But, of course, we have to decide who's on top of the ticket, and I think that the people of Ohio very clearly said that it should be me."

Not so fast, Obama replied. "I think it is very premature to start talking about a joint ticket," he told reporters on his plane.