WASHINGTON -- On Super Tuesday, John McCain secured the Republican nomination. How did that happen? Simple. In the absence of a compelling conservative, the Republican electorate turned to the apostate sheriff.

In the beginning, there were two. There was America's mayor, Rudy Giuliani, determined to "go on offense." And there was America's maverick, John McCain, scourge of Iraq wobblies. Both aroused deep suspicions among conservatives.

Mr. Giuliani's major apostasy is being pro-choice on abortion. Mr. McCain's apostasies are too numerous to count. He's held the line on abortion, but on just about everything else he could find -- tax cuts, immigration, campaign finance reform, Guantanamo -- he not only opposed the conservative consensus but insisted on doing so with ostentatious self-righteousness.

The story of this campaign is how many Republicans didn't care and felt that national security trumps social heresy. The problem for Messrs. Giuliani and McCain, however, was that they were splitting that constituency. Then came Mr. Giuliani's humiliation in Florida. After he withdrew from the race, he threw his support to Mr. McCain -- and took his followers with him.

Look at the numbers. Before Florida, the national polls had Mr. McCain hovering around 30, and Mr. Giuliani in the mid-teens. After Florida, Mr. McCain's numbers jumped to the mid-40s, swallowing the Giuliani constituency whole. On Super Tuesday, the Giuliani effect showed up in the big Northeastern states -- New York, New Jersey, Connecticut -- and California. Mr. McCain won the first three with absolute majorities of 51 percent or more. And in California, McCain-Giuliani (plus Schwarzenegger, for good measure) moderate Republicanism captured 42 percent of the vote.

Elsewhere, where Mr. Giuliani was not a factor, Mr. McCain got no comparable boost. In Oklahoma, Missouri, Arkansas, Tennessee, Alabama and Georgia, he could never break through even 37 percent. The vote was divided roughly evenly among Mr. McCain, Mike Huckabee and Mitt Romney (trailing). But these splits were not enough to make up for the winner-take-all big ones, all of which Mr. McCain won. The other half of the story behind Mr. McCain's victory is this: There would have been a far smaller Republican constituency for the apostate sheriff had there been a compelling conservative to challenge him. But there never was.

The first messianic sighting was Fred Thompson, who soared in the early polls, then faded because he was too diffident and/or normal to embrace with any enthusiasm the indignities of the modern campaign. Then, for that brief and shining Iowa moment, there was Mr. Huckabee -- until conservatives actually looked at his record (on taxes, for example) as governor of Arkansas, and listened to the music of his often unconservative populism.

That left Mr. Romney, the final stop in the search for the compelling conservative. I found him to be a fine candidate who would have made a fine president. But until very recently, he was shunned by most conservatives for ideological inauthenticity.

Then, as the post-Florida McCain panic grew, conservatives tried to embrace Mr. Romney, but the gesture was both too late and as improvised and convenient-looking as Mr. Romney's own many conversions. (So late and so improvised that it could not succeed. On Thursday, Mr. Romney withdrew from the race.)

Conservatives are on the eternal search for a new Reagan. They refuse to accept the fact that a movement leader who is also a gifted politician is a once-in-a-lifetime phenomenon. But there's an even more profound reason why no Reagan showed up this election cycle and why the apostate sheriff is going to win the nomination. The reason is George W. Bush. He redefined conservatism with a "compassionate" variant that is a distinct departure from classic Reaganism.

Mr. Bush muddied the ideological waters of conservatism. It was Mr. Bush who teamed with Teddy Kennedy to pass No Child Left Behind, a federal venture into education that would have been anathema to (the early) Reagan. It was Mr. Bush who signed the McCain-Feingold campaign finance reform. It was Mr. Bush who strongly supported the McCain-Kennedy immigration bill. It was Mr. Bush who on his own created a vast new entitlement program, the Medicare drug benefit. And it was Mr. Bush who conducted a foreign policy so expansive and, at times, redemptive as to send paleoconservatives like Pat Buchanan and traditional conservatives like George Will into apoplexy and despair (respectively).

Who in the end prepared the ground for the McCain ascendancy? Not Feingold. Not Kennedy. Not even Giuliani. It was George W. Bush. Bush begat McCain.

Mr. Bush remains popular in his party. Even conservatives are inclined to forgive him his various heresies because they are trumped by his singular achievement: He's kept us safe. He's the original apostate sheriff.