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John McCain tonight sealed the Republican nomination with a clean sweep of primary victories in Ohio, Texas, Vermont and Rhode Island.

Mike Huckabee, who had refused to duck out of the Republican race, conceded defeat at a campaign rally in Irving, Texas, while the White House confirmed that George W Bush will meet Mr McCain to formally endorse him.

“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,” Mr McCain told supporters in Dallas.

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Mike Huckabee told his his end-of-the-road rally: “We started this effort with very little recognition and virtually no resources. We ended with slightly more recognition and very few resources.

“But what a journey. A journey of a lifetime.”

The victory now allows Mr McCain to concentrate on November's presidential race and, later this month, he is due to meet Gordon Brown and David Cameron in London as he seeks to burnish his foreign policy credentials.

The trip will also include visits to Iraq and Israel, as Mr McCain begins a concerted effort to make the issues of Iraq and national security the dominant themes in his fight against the Democratic nominee.

Mr McCain is determined to base his campaign against the Democrats on the issue of national security, at a time when the faltering US economy is now the dominant concern for voters.

Mr McCain, a Vietnam War veteran, offers a starkly different Iraq policy to either Barack Obama and Hillary Clinton, who have vowed to remove nearly all combat troops from Iraq by 2009. The Arizona senator, at a time when more than two-thirds of Americans say the war was a mistake - and after it has significantly faded from the national conscience - is advocating yet another increase in troops “if we are to achieve long-term success in Iraq”. He said recently that if necessary, the US military should have a presence in Iraq for 100 years, a remark that has drawn scorn from his Democratic opponents.

Yet the power of the national security argument was evident this week, after a controversial advertisement by Mrs Clinton questioning Mr Obama’s readiness to be commander-in-chief dominated the Democratic race. In recent head-to-head matchups, Mr McCain beats both the Democratic candidates.

Referring to the advertisement, which asks voters who they would like answering a foreign policy emergency call in the White House at 3am, Mr McCain, 71, said on Monday: “I’m certainly the most experienced and qualified to answer that phone.” In front of reporters, he then touched on a series of foreign challenges, including the curtailment of democracy in Russia, violence in the Middle East and the threat of war in South America.

Domestically, Mr McCain must now turn his attention to his choice of a running mate, one of the first major and most important decisions a nominee makes. He must also try to assuage doubts over his competency and knowledge in economic matters - and his advanced age - vulnerabilities that may influence his Vice-Presidential pick. John Kerry, George W Bush and Bill Clinton all announced their choices of John Edwards, Dick Cheney and Al Gore in July, after a careful vetting process and a time of year when the news cycle is slow.

Mr McCain is likely to be considering Colin Powell and General David Petraeus as possible picks, but either would make the ticket top heavy in military acumen but very short of executive and economic expertise.

Another possible choice, and a woman who has received favourable reviews inside the Republican party, is Kay Bailey Hutchison, the Texas senator. Yet with Mr McCain’s 25 years on Capitol Hill — including over 20 in the Senate — another colleague with no executive experience could disqualify her. She has also said recently she does not want to be Vice-President.

Mr Huckabee could shore up support among religious conservatives, but his refusal to bow out of the nominating race until now has irked the McCain camp. Haley Barbour, the impeccably conservative, experienced and widely respected Mississippi governor would also be a popular choice on the Republican Right with his background as a former White House political director for Ronald Reagan.

One man bound to be on the shortlist is Charlie Crist, 51, the highly popular Florida governor and successor in the Sunshine State’s gubernatorial mansion to Jeb Bush. He endorsed Mr McCain early on in a critical swing state that once again this year will play an important role in the presidential election.

Two governors offer Mr McCain both youth and executive experience: Minnesota’s Tim Pawlenty, 47, whose state has been won by a Democrat in the last eight presidential elections but will be viewed by both parties as up for grabs this year, and South Carolina’s Mark Sanford, 47, is a conservative and fiscal hawk.

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