Senator Clinton broke Senator Obama’s 11-contest winning streak last night by defeating him soundly in Ohio and Rhode Island and scoring a narrow win in Texas as well. However, it was doubtful that her late-inning rally would give her enough momentum to erode the delegate advantage Mr. Obama has built up from his string of victories.

With three-fourths of Ohio precincts reporting, Mrs. Clinton led Mr. Obama, 56% to 42%, though the gap was expected to close somewhat as urban votes were counted.

In the night’s other big contest, in Texas, Mr. Obama had a narrow lead in the first results, but late last night, Mrs. Clinton passed him in the vote count. With 55% of precincts in, she led 50% to 48%.

The Democratic hopefuls split the two smaller states at stake yesterday. In Vermont, Mr. Obama won handily, 60% to 38%. Mrs. Clinton took Rhode Island by a similar margin, 58% to 40%.

However, the healthy win for Mrs. Clinton in Ohio and the slim victory for her in Texas could still leave Mr. Obama with more delegates on the day. One-third of Texas’s delegates were selected yesterday through caucuses, where the Illinois senator has tended to outperform his colleague from New York.

Meanwhile, Senator McCain of Arizona swept all four states voting yesterday, surpassing the tally of 1,191 delegates needed for the Republican presidential nomination. He was expected to join President Bush for lunch today and receive a formal endorsement.

While President Clinton said two weeks ago that Mrs. Clinton needed to win both Texas and Ohio to be the nominee, her campaign has retreated from that stance in recent days, arguing that a victory in either state would be enough for her to press on.

“You know what they say, as Ohio goes, so goes the nation. Well, this nation’s coming back and so is this campaign,” an effusive Mrs. Clinton told a rally in Columbus last night. “The people of Ohio have said it loudly and clearly we’re going on we’re going strong and we’re going all the way.”

In San Antonio, Texas, Mr. Obama congratulated Mrs. Clinton on her wins in Ohio and Rhode Island, though he did not concede defeat in the Lone Star State. “We know this: no matter what happens tonight we have nearly the same delegate lead as we did this morning and we are on our way to winning this nomination,” he said.

In his speech last night, Mr. McCain seemed to set his sights on Mr. Obama by declaring that the vote to go to war in Iraq should not be central issue in the fall campaign. “It is of little use for Americans for their candidates to avoid the many complex challenges of these struggles by re-litigating decisions of the past,” he told backers in Dallas. “I will defend the decision to destroy Saddam Hussein’s regime as I criticized the failed tactics that were employed for too long to establish the conditions that will allow us to leave that country with our country’s interests secure and our honor intact. But Americans know that the next president doesn’t get to re-make that decision.”

Mr. McCain also recalibrated his message on Iraq, promising “to bring that war to the swiftest possible conclusion without exacerbating a sectarian conflict that could quickly descend into genocide.”

Another Republican candidate, Michael Huckabee, withdrew last night, belatedly acknowledging that Mr. McCain’s nomination was inevitable. “I extended to him not only my congratulations but my commitment to him and to the party to do everything possible to unite our party but, more importantly, to unite our country,” the former Arkansas governor told a rally in Irving, Texas.

Mr. Obama’s difficulties in Ohio and elsewhere came as his presidential campaign struggled with its first sustained spate of negative press coverage. For the past several days, news reports in the area were dominated by stories that a top economic adviser to Mr. Obama’s campaign, Austan Goolsbee, told Canadian officials that the Illinois senator was posturing when he talked of making major changes to the North American Free Trade Agreement. Mr. Obama flatly denied the initial reports, which did not identify Mr. Goolsbee. However, the Obama camp later backtracked, acknowledging the encounter but insisting that it was not “a formal meeting” and that the Canadians misunderstood the University of Chicago economics professor.

Exit polls showed that the Ohio primary mimicked the cleavages seen among Democratic voters in the Super Tuesday contests last month. Mr. Obama dominated the youth vote, winning 67% of those under 30, while Mrs. Clinton swept seniors, taking 70% of those over 65. The former first lady got 55% of those with no college degree, while the Illinois senator got an equal percentage of those who graduated from college. Mrs. Clinton did better with wealthier voters than she has in earlier fights.

Given the marked splits, the demographics of Ohio seemed to tilt the state to Mrs. Clinton. Almost two-thirds of those voting lacked college degrees, while voters older than 50 outnumbered those under 30 three-to-one.

In Texas, there was direct evidence that Mr. Obama’s juggernaut has stalled. Voters who decided in the last three days broke for Mrs. Clinton, 61% to 38%, the exit polls indicated.

Mrs. Clinton’s hopes of winning the nomination now seem to rest on two possibilities. In one scenario, a catastrophic error or event involving Mr. Obama’s campaign drives him from the race or prompts a huge number of voters in remaining states to abandon him. Another scenario involves the so-called superdelegates handing the nomination to Mrs. Clinton even though Mr. Obama leads in the tally of delegates elected through caucuses and primaries.

While Mr. Obama’s difficulties in the past few days stemming from the NAFTA issue and a fraud trial for one of his supporters could be considered the political equivalent of a bad head cold, for Mrs. Clinton to pull even in the pledged delegate count she needs Mr. Obama to come down with something closer to political pneumonia.

The prospect of a superdelegate-driven win is also remote, in part because it would be so grim. A number of prominent Democrats have said it would be wrong for the superdelegates to override the will of the people as distilled by the results of the primaries and caucuses. A superdelegate who ran Vice President Gore’s presidential campaign in 2000, Donna Brazile, has vowed to quit the party if this happens.

Mrs. Clinton’s most realistic hope may be to close the pledged delegate gap to the point where her advisers can reasonably spin the race as a tie. However, Mr. Obama’s advisers are already indicating that if he holds even a small lead, the result should be respected.

“What is the number they’re comfortable suggesting where the superdelegates might be willing to overturn the will of the people?” Mr. Obama’s campaign manager, David Plouffe, told reporters on Monday. “They need to walk through the states and show folks how they either are going to retake the pledged delegate lead or be honest about what number they think is acceptable if they still have a deficit.”

The Clinton campaign has not responded with a number, except to say that either candidate will need superdelegates to clinch the nomination, that those delegates should vote their conscience, and that they should not feel compelled to track the primary and caucus results.

Speaking to reporters on his campaign plane before the polls closed yesterday, Mr. Obama indicated he did not see how Mrs. Clinton could gain the upper hand in the nominations contest. “My head tells me is that we’ve got a very sizable delegate lead that is going to be hard to overcome,” he said. “But look, she is a tenacious and determined candidate and so we’re just going to make sure we work as hard as we can as long as it takes.”

The next Democratic contests are in Wyoming on Saturday and in Mississippi on March 11, followed by a hiatus of six weeks before the Pennsylvania primary on April 22.