Contrary to all the flak he’s taking for it, Sen. John McCain has every right to “go negative” on Sen. Barack Obama — but he’d better firm up his positive message, too.

McCain has been making a case that lower corporate taxes will create “jobs for America” and that “all of the above,” including nuclear power and offshore drilling, is the way to go on energy. But he’s still short of enunciating a comprehensive vision to compete with Obama’s.

Meantime, his jabbing at Obama’s “celebrity,” and using images of Paris Hilton and Britney Spears in TV ads, was juvenile. But it was scarcely “racist” or “Rovian,” as many critics charged.

It has become standard among Democrats to accuse Republicans of “smearing” or “sliming” their candidates whenever the GOP goes negative.

“Willy Horton,” “Swift Boat” and “Karl Rove” are shorthand for Democratic accusations, and the words alone are widely accepted as proof of GOP dirty tricks.

Republicans certainly did exploit 1988 Democratic candidate Michael Dukakis’ furlough of convicted murderer Willie Horton, but it is a fact that Horton committed rape and assault after his release, reinforcing doubts about Dukakis’ stance on crime.

In 2004, the Swift Boat Veterans for Truth did have a legitimate beef against Sen. John Kerry, D-Mass., who, as an antiwar veteran, once charged that atrocities were routinely committed by U.S. troops in Vietnam.

And while somebody surely inspired underhanded attacks on McCain in South Carolina in 2000, on Sen. Max Cleland, D-Ga., in 2002 and on 2006 Tennessee Senate candidate Rep. Harold Ford Jr. (D), no one has ever established that Rove, political guru to George W. Bush, was responsible.

In fact, of all the negative ads run in recent elections, the worst actually was run against Bush in 2000, in which the NAACP charged that the then-Texas governor’s veto of a hate crimes bill was tantamount to condoning a racist murder.

In this year’s election, neither McCain nor Obama has been anywhere near as raw — and, hopefully, won’t be. Each has hit below the belt, though.

McCain charged that Obama “would lose a war to win an election.” Obama’s policies would have lost the Iraq War, but McCain can’t prove they were politically motivated.

McCain charged that during Obama’s trip to Europe, he chose to play basketball rather than visit wounded troops, whereas the circumstances of Obama’s choice are actually complicated and murky.

Meantime, Obama falsely charged that McCain wanted to conduct a “100 years war” in Iraq and that Republicans would use race as an issue against him, which McCain has never done.

The fact is that Obama’s fitness to be chief executive and commander in chief is probably the major question in the minds of swing voters — and McCain has every right to reinforce their doubts.

Obama’s youth, inexperience, judgment, values and consistency are all legitimate targets for Republicans, and, obviously, so are his policies.