Obama, appearing disappointed, insisted the contest was still his to lose, citing his continuing edge in the delegate count. "We know this: No matter what happens tonight, we have nearly the same lead as we did this morning, and we are on our way to winning the nomination," the Illinois senator told cheering fans at a late-night rally in San Antonio.

The Democrats split two other contests, Clinton winning handily in Rhode Island and Obama easily taking Vermont.

On the Republican side, John McCain swept the day's four GOP primaries to clinch the Republican nomination and force Mike Huckabee from the race. He and the two Democrats called McCain to offer congratulations.

The Arizona senator, who staunchly supports the U.S. presence in Iraq, signaled his intention to make the war a major issue in the fall. "Our most vital security interests are involved there," McCain told supporters in Dallas.

He planned a stop at the White House today to receive the endorsement of President Bush, who beat McCain in his 2000 try for the White House. The two have had a tepid relationship ever since.

The Democratic race, by contrast, appears deeply unsettled after a long night of counting ballots in Ohio and Texas. The next major contest is April 22 in Pennsylvania, making for an unusually long stretch between races in this heavily compacted election season.

Wyoming will hold caucuses on Saturday, and Mississippi has a primary Tuesday. But neither is likely to reshape the essential dynamic of what has become the most competitive presidential nominating fight in at least 40 years. That promises at least another seven weeks of intensive campaigning.

At a raucous victory party in Ohio's capital city, Clinton told a ballroom full of backers: "As Ohio goes, so goes the nation. Well, this nation's coming back and so is this campaign. People of Ohio have said it loudly and clearly: We're going on, we're going strong and we're going all the way."

Tuesday was the biggest day of balloting left on the Democrat's election calendar -- with 370 pledged delegates at stake in four contests -- and voters responded as they have throughout the campaign, turning out in record numbers

In Ohio, they braved raw, late-winter weather, including freezing rain in the north and flood warnings across most of the state. Citing the conditions, and a shortage of paper ballots in some precincts, state officials went to court to extend the polling hours in several places.

Obama, bidding to become the nation's first black president, entered the day with 11 straight victories and hoped to force Clinton from the race by taking Ohio and Texas. Clinton's top advisors -- including her husband, the former president -- had said she needed victories in both states to remain viable.

But on Tuesday the New York senator seemed to back away from that assessment, intimating earlier in the day that a victory in Ohio alone could persuade her to continue, even if Obama maintained his lead in delegates. She is vying to become the first female president in the nation's history.

"My husband didn't get the nomination wrapped up until June. That has been the tradition," Clinton told reporters, though she failed to note that the primaries when he first ran in 1992 were much more spread out. As of today, well over half the states have voted, awarding more than 80% of delegates to the party's national nominating convention.

Even before the first polls closed, Obama was bracing for the race to continue, citing plans to campaign ahead of the contests in Wyoming and Mississippi. "Either way, we'll go on," he told reporters in San Antonio.

On the Republican side, McCain began the day all but certain to clinch the GOP nomination and eliminate Huckabee. Aides printed up a banner with the number of delegates needed -- 1,191 -- and hung it in a ballroom at Dallas' Fairmont Hotel. (It was kept covered until McCain officially went over the top.)

Huckabee dropped out of the race less than an hour after the last polls closed in El Paso. Addressing his supporters in Irving, Texas, the former Arkansas governor said that he had called McCain moments earlier to congratulate him and offer "to do everything possible to unite our party" and "unite our country so that we can be the best nation we can be."

A short time later, McCain addressed supporters in Dallas with his wife, Cindy, beaming by his side. McCain claimed the nomination, just a few months after many had declared him politically dead, "with confidence, humility and a great sense of responsibility."

Signaling the broad outlines of his fall campaign, McCain scolded the two Democrats for opposing the war in Iraq, accused them of offering "big-government" solutions and, in a veiled shot at Obama's speechifying, suggested that Americans "aren't interested in an election that offers platitudes instead of principles."