Senator John McCain's rout of Governor George W. Bush in New Hampshire this week has shocked Republicans, who had seen the primary elections as a simple triumphal procession towards the eventual "coronation" of Mr Bush as the man to recapture the White House.
Now McCain, with his unorthodox message and in-your-face campaign methods, has stirred up a fierce ideological debate inside the Republican Party over which man is the true blue conservative worthy of the Ronald Reagan legacy.
Last year the conservative wing of the party was at first dubious about Bush's slogan of "compassionate conservatism", which for them smacked too much of the "I feel your pain" squishiness of President Clinton.
But as millions of dollars poured into the Bush war chest and he soared in the polls, such doubts were swept aside and the party establishment dreamed about a second Bush in the White House.
McCain has challenged that rosy scenario and raised serious doubts about whether young Bush has the leadership credentials for the job and the right message to win. McCain swept the board in New Hampshire with virtually every category of voter - men, women, young, old, graduates, non-graduates, even conservatives, though by a smaller margin.
This is a real shock for the Republican establishment, which regards the colourful maverick's crusade for campaign finance reform and his anti-tobacco companies stance as heretical to Republican principles. McCain's taxation policies also seem at variance with Reagan era supplyside economics, which called for across-the-board tax cuts.
Bush and McCain are now criss-crossing South Carolina, where the next Republican primary takes place on February 19th, each proclaiming his conservative credentials. Bush headed first for a right-wing bastion of fundamental Christianity, the Bob Jones University, which awarded a doctorate to the Rev Ian Paisley.
In a speech to about 6,000 students, Bush managed to use the word "conservative" six times in one minute. "It's a different world down here from New Hampshire. This is a state that embraces conservative values. I'm the conservative candidate."
But McCain insists he is also a conservative and his voting record in the Senate on gun control, the minimum wage and discrimination against gays and lesbians bears this out. The two men also want more defence spending.
It is on taxation that he and Bush have become entangled about who best represents conservative values in domestic policies.
Bush's proposed tax cut of $483 billion is over twice the size of McCain's, yet Republicans in New Hampshire, who are notoriously anti-tax, rejected Bush. McCain says he is the real conservative because he would use more of the huge budget surpluses that are beginning to pile up as the economy booms to save the state pensions scheme from eventual bankruptcy and pay down the national debt, which runs into trillions.
Bush's message has been that if you leave unspent surpluses in Washington, the government will find ways to spend them. So genuine conservatives should give the maximum back to the taxpayers. But in these times of unprecedented prosperity, voters seem less concerned with big tax cuts than with preserving their social security benefits when they retire.
McCain not only proposes smaller tax cuts than Bush, but he would use them to redistribute income from the very rich to those at the bottom of the scale and to middle-income earners. This produced gasps at the Wall Street Journal and cries of "heresy" among conservative ideologues. The liberal magazine New Republic, on the other hand, put McCain on the cover with the headline "This Man Is Not A Republican".
A Wall Street Journal columnist writes that McCain has "redefined what it means to be a conservative in a time of budget surpluses". McCain has "ushered in a referendum on what Republicanism and conservatism is to mean in the year 2000".
The president of the libertarian Cato Institute, Mr Ed Crane, says McCain's big win in New Hampshire is an indication that the Republican Party has "lost its philosophical moorings". Ronald Reagan's former political adviser Lyn Nofziger says of McCain: "I don't think he's a Reagan conservative. He's evolving and I think he's evolving leftward."
The Bush campaign, still shaken from the New Hampshire debacle, is having to redefine its strategy vis-a-vis McCain, just as Vice-President Al Gore has had to do to head off the challenge from former senator Bill Bradley.
Bush has started to harden his stance and portray McCain as a liberal masquerading as a Republican, but this risks being driven to the right and undermining his "compassionate conservatism" slogan. If he does beat McCain for the Republican nomination - and the betting still is that he will - Bush could find himself fighting the election next autumn from a more right-wing stance than he had planned.
President Clinton showed the Democrats that to win the White House you have to soften the ideological edges and move to the centre to attract the swing voters from the suburbs and the middleclass "soccer moms".
The equivalent task for the Republicans this year will be to show that they will not leave the less well off behind while giving tax cuts to the very rich. Bush believed he had found the right formula with his vague "compassionate conservatism" pitch, and until New Hampshire it seemed to be working.