With no end in sight, Democrats braced Wednesday for a prolonged and nasty fight for their party's presidential nomination as Barack Obama assailed his reinvigorated rival over taxes and, in a turnabout, her experience in foreign affairs.
Hillary Rodham 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 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 be 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 finalized 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 that 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 McCain, the presumptive Republican nominee, sat down at the White House for a hot-dog lunch with President Bush, followed by an endorsement in the Rose Garden.
The bitterness of the Democrats' race sparked questions on whether it might undermine the eventual nominee, enabling the Arizona senator to raise money and rally Republicans while Clinton and Obama effectively did his dirty work.
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.
"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 Wednesday.
For her part, Clinton lost no time renewing her accusation that Obama had overstated the significance of his opposition to the U.S. invasion of Iraq.
"Sen. 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 notably more aggressive in the aftermath of Tuesday's losses.
His campaign manager, David Plouffe, told donors in a fundraising e-mail that Clinton "stood shoulder to shoulder with McCain on the worst foreign-policy disaster of our generation" by voting to approve the U.S. invasion of Iraq.
Obama's team also 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."
"In the face of her unwillingness to release her tax returns," an Obama campaign memo said, "Hillary Clinton has made the false case in this campaign that she is more electable because she has been fully vetted. When it comes to personal finances, Sen. Clinton's refusal to release her [tax] returns denies the media and the American people the opportunity to even begin that process."
Underscoring its newly aggressive tack, Obama's campaign even circulated a memo documenting outrage over Clinton's remarks on the dearth of women in public office in Mississippi, where Democrats will hold a primary Tuesday. "How can Iowa be ranked with Mississippi?" she was quoted as saying.
In the memo, former Mississippi Gov. Ray Mabus, an Obama supporter, faulted Clinton for "a disturbing pattern of writing off and criticizing states that she's lost or that she doesn't expect to do well in, including small states and Southern states."
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."
Obama blamed his losses Tuesday in part on tougher media coverage that he said had resulted from Clinton's complaints of unfair treatment.
The assertion was crystallized by a pair of "Saturday Night Live" skits that spoofed the political press corps' supposed love affair with Obama.
"Hopefully now people feel like everything's evened out, and we can start actually covering the campaign properly," he said.
For all the rancor, questions arose Wednesday about whether Clinton and 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.
In nearly complete results, Clinton won the Ohio primary 54% to 44%, the Texas primary 51% to 47% and the Rhode Island primary 58% to 40%.
In the Vermont primary, Obama defeated her 59% to 39%.
Clinton and Obama will compete again Saturday in the Wyoming caucuses, followed by the Mississippi primary Tuesday.
But the next big contest will be the Pennsylvania primary on April 22. Also ahead in May and June are contests in Indiana, North Carolina, West Virginia, Kentucky, Oregon, Montana, South Dakota and Puerto Rico. In all, about 20% of pledged delegates remain to be chosen.
Ultimately, the contest could be settled by superdelegates, the roughly 800 party and elected officials who will cast ballots at the Democratic National Convention.
As he built momentum throughout February, Obama steadily eroded Clinton's lead in superdelegates, which has helped her narrow the overall margin.
As of Wednesday evening, Obama was leading Clinton 1,567 delegates to 1,462, according to the Associated Press. It takes 2,025 to win the nomination.
Finnegan reported from Washington, Barabak from Austin, Texas. Times staff writer Maria L. La Ganga contributed to this report from San Antonio and Chicago.