John McCain clinched the Republican presidential nomination on Tuesday with four big victories that drove his last major rival, former Arkansas Governor Mike Huckabee, out of the race.
The wins in Vermont, Ohio, Texas and Rhode Island gave McCain more than the 1,191 delegates needed to win the nomination, according to media delegate counts. President George W Bush was expected to endorse the Arizona senator at the White House on Wednesday.
"I will do everything possible to unite our party," Huckabee told supporters in Irving, Texas, saying he had called McCain to congratulate him on capturing the nomination.
In the Democratic race, Barack Obama scored an easy win in Vermont and rival Hillary Clinton won Rhode Island as she battled to save her presidential candidacy on Tuesday in their big-state showdowns in Ohio and Texas.
Clinton's Rhode Island victory broke Obama's string of 12 straight victories in their hard-fought duel to be the Democrat who squares off against McCain in the November presidential election.
Clinton, a New York senator, was under pressure to score wins in the two biggest states, Ohio and Texas, to keep her White House hopes alive and prolong the hotly contested Democratic campaign with Obama.
Turnout was heavy in all four states, and the Democratic campaigns of Obama, an Illinois senator, and Clinton traded accusations of irregularities at the polls in both Ohio and Texas. An Ohio judge granted a request from the Obama campaign to extend voting in some Cleveland-area precincts.
Few returns were available yet in Ohio, and Obama had a lead in slight, early returns from Texas.
For Clinton, a New York senator, wins in both Ohio and Texas would rejuvenate her campaign and send the race on toward the next major contest -- Pennsylvania on April 22.
Losses in both, or possibly even one, could set off a stampede of party support for Obama, raise pressure on Clinton to drop out and make it even tougher for her to cut Obama's lead in the pledged delegates who will choose the Democratic nominee to contest November's presidential election.