The most momentous political story of the hour is not what you might think. It has to do with an American politician now living in London and his aspiration to become president of America.

His name is Boris Johnson. He, an exemplary conservative, has just beaten one of the most rebarbative left-wing reactionaries in Britain to become the mayor of London.

Mr. Johnson ran a very fine campaign, an amalgam of high intelligence, sound principle, rollicking good humor, and energy that could be branded New Tory. Mind you, New Toryism will arrive on these shores in due course.

Presidential aspirants often are accused of pursuing office with the intent of using that office as a stepping stone to a still higher office. The wife of a former Arkansas governor, when running for a Senate seat in New York in 2000, was accused of intending New York to be her “stepping stone” to the presidency. Her husband too was accused of using his reelection to the governor’s mansion as a stepping stone to the White House: months after his reelection, Boy Clinton’s 1992 presidential campaign began. For that matter, the New York Senate seat that Hillary now holds is the same seat once held by Robert Kennedy, who also was accused by opponents of intending to use the seat as his stepping stone to the highest office in the land. Incidentally, both Mrs. Clinton and Kennedy came to New York as outsiders, she from Arkansas, he from Massachusetts. Consequently, both suffered the charge of being called carpetbaggers.

So using a governorship or a Senate seat as a steppingstone to the presidency is not new. Using City Hall in London is. Geographically speaking, Mr. Johnson’s presidential campaign will make him the most ambitious carpetbagger in American history. He was born in New York General Hospital, New York, N.Y., on June 19, 1964 — the year remembered by American conservatives as the Goldwater Year.

It is now faintly circulating through the American press that Mr. Johnson was born here, but so is the report that he gave up his citizenship in 2006 after encountering passport problems with fussy American immigration authorities. The report is in error.

I can now disclose that the American Spectator, in another of its world exclusives, has discovered that the newly elected mayor of London never terminated his citizenship. The full report is in the June issue. He is as American as Barack Obama.

The confusion arises because of a comical piece Mr. Johnson wrote in the August 9, 2006, issue of the Spectator of London. In it he reported his rude encounter with our immigration authorities and his vow to give up his American citizenship. But wait — now I can report that when he presented himself at the American embassy to terminate his American citizenship he good-naturedly changed his mind. The procedure threatened to become too expensive in terms of tax liabilities alone. When Mr. Johnson sets out for the Republican nomination, there will be no doubt as to where he stands on tax cuts.

Already Mr. Johnson’s presidential ambitions are being circulated in the British press. Apparently he has joked about his plans for years. This week, Stuart Reid, a confidant of Mr. Johnson’s at the British Spectator, which Mr. Johnson then edited, has written that Mr. Johnson will not actually launch his campaign until 2016. Mr. Reid believes Mr. Obama will win the presidency this fall.

I doubt Mr. Obama will defeat Senator McCain, and readers of this column might recall that one year ago in “The Clinton Crack-Up” I predicted Mrs. Clinton’s faltering before a challenge from the Democratic Party’s younger generation. My prediction came at a time when such political savants as Dick Morris were touting Mrs. Clinton as the “inevitable” nominee and next president.

Today I predict that Mr. Johnson, working from the City Hall of London, will have a salubrious influence on conservatives both in Britain and America. His campaign for the American presidency will begin long before Mr. Reid speculates that it will, and it cannot begin too soon for me.

I say Mr. Johnson will be a salubrious force because I have known him since his tenure as the editor of the Spectator. He brings to conservatism something it has lacked, at least on this side of the Atlantic, since the presidency of Ronald Reagan and the retirement of William F. Buckley. Frankly, it is my kind of conservatism: libertarian, admiring tradition, and employing government only in those areas where government is needed. After his stint in journalism, where he was superb, Mr. Johnson entered Parliament. There he was Thatcherite, but with beneficent bacteria of skepticism, irony, and subversion.

All of this comes together on the campaign trail, where he is a refreshing contrast to the solemn blowhards. Campaigning in upscale Henley he joshed about his Conservative Party’s excessively grim slogan — “You’ve paid your taxes. So where are the police?” — employing his own whimsical alternative: “You’ve paid your taxes. So where are the tennis courts?”

Campaigning in 2004 he famously declared, “Voting Tory will cause your wife to have bigger breasts and increase your chances of owning a BMW M3.” Now after all these months of Barack and Hillary’s poppy and cock, imagine the heap Mayor Johnson would leave them in. It is only a matter of time before he returns to his native land and saves conservatism from Newt Gingrich.

Mr. Tyrrell is the founder and editor in chief of the American Spectator, a contributing editor of the New York Sun, and an adjunct scholar at the Hudson Institute.