John McCain clinched the Republican presidential nomination on Tuesday and Democrat Hillary Clinton won a crucial showdown with rival Barack Obama in Ohio to breathe new life into her campaign and prolong the Democratic race.
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 greet the Arizona senator at the White House on Wednesday and back his campaign.
"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 the November presidential election.
The win for Clinton, 60, a New York senator, is almost certain to send the hotly contested Democratic race on toward the next major contest -- Pennsylvania on April 22.
The Ohio victory represented a major comeback for Clinton, who was under pressure to win in the two biggest states, Ohio and Texas, or face calls to withdraw from the race.
"We're going on, we're going strong, and we're going all the way," Clinton told roaring supporters in Columbus, Ohio. "We're just getting started."
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.
She still faces a sizable deficit to Obama in delegates to the August nominating convention, and appeared unlikely to close the gap significantly on Tuesday unless she wound up with big winning margins in Ohio and Texas.
Clinton led Obama slightly, 50 percent to 49 percent, with about 40 percent of the precincts counted in Texas.
Clinton also captured Rhode Island and Obama scored an easy win in Vermont. Her Ohio and Rhode Island victories broke Obama's string of 12 straight wins in their hard-fought duel to be the Democrat who squares off against McCain.
Turnout was heavy in all four states, and the Democratic campaigns of Obama, 46, an Illinois senator, and Clinton traded accusations of irregularities at the polls in both Ohio and Texas.
Mccain looks to November
In his victory speech, McCain took aim at both of his likely Democratic opponents and criticised their pledges to revisit US trade treaties, punish companies that send jobs overseas and withdraw US troops from Iraq.
"The next president must explain how he or she intends to bring that war to the swiftest possible conclusion without exacerbating a sectarian conflict that could quickly descend into genocide, destabilising the entire Middle East," said McCain, who received a call of congratulations from Obama.
McCain has had trouble winning over conservatives unhappy with his views on immigration, his past opposition to Bush's tax cuts in 2001 and 2003, and his criticism of some religious conservative leaders as "agents of intolerance" during his failed 2000 presidential campaign.
Huckabee said he called McCain to congratulate him and promised to actively back his candidacy and rally support among Republicans. "I will do everything possible to unite our party," Huckabee told supporters in Irving, Texas.
In the Democratic showdowns, a cautious Obama said before the results were announced that he did not believe Clinton could catch him in the race for delegates.
"What my head tells me is we've got a very sizable delegate lead that is going to be hard to overcome," he said.
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.
An MSNBC count gave Obama 1,194 delegates to Clinton's 1,037 before Tuesday's showdowns, well short of the 2,025 needed to win the nomination.
Like her husband, former President Bill Clinton, who nicknamed himself "The Comeback Kid" for his improbable rise to the White House in 1992, Clinton has dodged disaster before.