The victories for Clinton, a New York senator, snapped Obama's winning streak at 12 and defied widespread predictions that defeats in Ohio and Texas would force her out of the White House race.

The hard-fought Democratic presidential duel moves on to contests in Wyoming and Mississippi and the next major showdown in Pennsylvania on April 22, with Clinton still trailing Obama in the pledged delegates who will choose the nominee at the August convention.

"We're going on, we're going strong, and we're going all the way," Clinton, 60, told roaring supporters in Columbus, Ohio. "We're just getting started."

McCain's four big victories in Vermont, Ohio, Texas and Rhode Island drove his last major rival, former Arkansas Gov. Mike Huckabee, out of the race and gave McCain more than the 1,191 delegates needed to win the nomination.

President George W. Bush will endorse the Arizona senator at the White House on Wednesday, capping McCain's comeback from the political scrap heap last year when his campaign was down in the polls and counted out.

"I am very pleased to note that tonight, my friends, we have won enough delegates to claim with confidence, humility and a sense of great responsibility that I will be the Republican nominee for president of the United States," McCain, 71, told supporters in Dallas.

"The contest begins tonight," the former Navy fighter pilot and prisoner of war in Vietnam said, looking ahead to a match-up with either Obama or Clinton in November.

Exit polls showed Clinton won big among voters who decided in the last few days, when she questioned Obama's readiness to be commander in chief and the sincerity of his pledges to renegotiate the North American Free Trade Agreement, which is blamed in Ohio for manufacturing job losses.

Under Democratic rules allowing the losers in each state to win a proportional amount of delegates, Clinton must win many of the remaining contests by big margins to close the delegate gap behind Obama.