Republican Senator John McCain won all four presidential primary votes on Tuesday and American news organizations now estimate he has clinched the Republican Party's presidential nomination. In the Democratic race, Senator Barack Obama won the primary in Vermont, Senator Hillary Rodham Clinton has been declared the winner of Ohio, one of two crucial delegate-rich states being contested. VOA National Correspondent Jim Malone has more on the McCain victory from Washington.

John McCain won a sweep of the Tuesday primaries, including votes in Vermont, Rhode Island, Ohio and Texas.

News organizations estimate he now has more than 1,191 delegates he needs to secure the Republican Party's presidential nomination.

McCain addressed jubilant supporters in Texas after his main remaining rival, former Arkansas governor Mike Huckbee, conceded McCain will be the Republican nominee.

"So stand up with me, my friends, stand up and fight for America, for her strength, her ideals and her future," said John McCain. "The contest begin tonight!"

Huckabee spoke briefly with McCain by phone and then told his supporters in Texas it is time to rally behind McCain and defeat the Democrats in November.

"I extended to him not only my congratulations, but my commitment to him and to the party to do everything possible to united our party, but more importantly, to united our country," said Mike Huckbee.

McCain was expected at the White House Wednesday, where President Bush will offer his formal endorsement.

McCain's clinching of the nomination caps a remarkable political comeback. McCain was the early frontrunner for the Republican nomination, but dropped badly in the polls, last year, amid fundraising difficulties and staff upheavals.

But McCain rallied his campaign with a victory in the New Hampshire primary, in early January, and fought his way to the nomination, despite some continued grumbling from conservative activists about his views on immigration and campaign finance reform.

McCain faces some major challenges as the apparent Republican nominee. He will head into an election where a majority of Americans say they want change and disapprove of the job done by President Bush.

Concern over the weakening U.S. economy has emerged as perhaps the major issue in the presidential campaign and that usually works to the advantage of Democrats.

But McCain argues that he has more experience on national security and foreign policy matters than either Senator Obama or Senator Clinton. McCain has also tied his political fate to the success of the military surge strategy in Iraq, which has produced some positive results in recent months.

McCain's victory came on his second try for the Republican nomination. He lost out to then-Texas governor George W. Bush in 2000, and Mr. Bush went on to narrowly defeat Democrat Al Gore in that year's election.