As the scope of Senator John McCain's dramatic victory in the Michigan primary election emerged during the vote counting, the stunned supporters of Governor George W. Bush derided the result as a "hostile take-over" bid for the Republican Party.

Allowing for the heightened emotions of an election night, there was still some truth in the charge. The maverick senator from Arizona had won a Republican primary with only a quarter of the Republican votes cast. He won only because Michigan Democrats and independents turned out in large numbers to vote for him.

As Mr Bush said ruefully as he flew off to California, "I won overwhelmingly in double digits among Republicans."

Something very curious happened in Michigan which has not been seen before in American politics in an election year. It may yet split the Republican Party which has been betting heavily on Mr Bush to re-conquer the White House after eight years of the Clinton-Gore presidency.

More non-Republicans than registered Republicans turned out to vote in Michigan which, to an outsider and even to many Americans, seems a bit crazy. A Republican primary election is held in each state to allow party members to choose their candidate to run in the presidential election in November, so why are Democrats muscling in on this process?

The reason is that the Republican Party in a number of states had decided to hold "open primaries" hoping that they would attract conservative Democrats and independents to the party by letting them join in their selection process.

This has turned out to be a Godsend for McCain who has been running as the underdog against a Bush campaign which has raised $73 million and is supported by the party establishment. McCain has won in New Hampshire and Michigan thanks to the votes of Democrats and independents.

For the Bush campaign this is a machiavellian ploy by Democrats to sabotage Bush's "coronation" as the only Republican who can win back the White House which Bill Clinton snatched from his father in 1992. These "crossover" Democrats will nip back to their own party to vote for Vice-President Al Gore or Bill Bradley come the presidential election, the Bush campaign complains.

"John McCain is not party building, he's party borrowing," said Governor John Engler of Michigan, who had promised Bush an "asbestos firewall" in the state.

Not so, says McCain, whose campaign has been revived with the Michigan and Arizona victories after his drubbing last week in South Carolina. McCain insists that he is genuinely attracting conservative Democrats and young independent voters - in the way that Ronald Reagan did in the 1980s.

McCain correctly points out that Republicans are a permanent minority in today's America and can never win the presidency without attracting non-Republicans, both the kind who were known as Reagan Democrats and first-time voters who will make up their own minds on policies. McCain also cites a poll this week which shows that he would beat Gore in the election by 59 to 35 points, whereas Bush would win over Gore by only 50 to 45 points. In other words, McCain is "more electable" this year than Bush.

Bush naturally disputes this and points out that McCain's heyday with open primaries is over. So far only 10 per cent of the delegates for the Republican convention have been selected but on March 7 th, or Super Tuesday as it is known, 30 per cent will be decided in one day alone, mainly in California and New York where only registered Republicans can vote for delegates.

McCain knows he cannot win the Republican nomination without majority party support and he has already begun his wooing of the party faithful. From Arizona yesterday he proclaimed: "I am a proud Reagan conservative. I love the Republican Party. It is my home."

In an amazing twist in this Republican campaign, Bush began by calling himself a "compassionate conservative" in an obvious attempt to soften the hard edges of the party's image, promising to reach out to Hispanic and African-American minorities. But as the pressure came on from McCain, the Texas governor was forced to the right, especially in South Carolina where the Christian conservatives rallied to him in large numbers.

McCain's record after 17 years in the Senate was hard-edged conservative on issues like gun control, taxation, abortion and the environment. He angered most of his Republican colleagues, however, by pressing for reform of the campaign finance system through which most of them get elected. He angered them further by denouncing the "iron triangle" in Washington where big money, lobbyists and legislators combine to corrupt the political system.

These McCain reforms have now been shown to have a strong appeal to non-Republicans and Bush has tried to steal some of his rival's clothes by calling himself a "reformer with results", pointing to his good record on education in Texas.

The problem for Bush now is that while he can win the hardcore Republican vote and may yet repulse the McCain challenge in the remaining primaries, he has to show he can reach outside the party to become electable. Six months ago, the polls showed that Bush would crush Al Gore in a presidential contest, but this was before he had to fight the bruising battles with McCain in New Hampshire, South Carolina and Michigan of the past three weeks.

On policies, the gap between Bush and McCain is not that big and will become smaller as McCain seeks to buttress his Republican credentials. What Bush must fear is McCain's personal appeal as a Vietnam war veteran whose heroism in captivity is inspirational for the thousands who are reading about it in his book, Faith of My Fathers.

After the sleaze associated with the Clinton era, both in fundraising and personal behaviour, McCain is tapping into a widespread feeling for old-fashioned patriotism and idealism. Even liberal Democrats who have little time for his conservatism on domestic issues, find something attractive in the 63-year-old former navy pilot who jokes his way around America with reporters on a bus called The Straight Talk Express, spurning the public relations spin behind which other candidates often shelter.

"In November I'll beat Al Gore like a drum," cries McCain after each win. "I'm Al Gore's worst nightmare."

He is also George Bush's.