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

[IMGCAP(1)]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 anti-war 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 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.

Questions about Obama’s fitness are certainly what’s keeping the 2008 race close, despite overwhelming voter hunger for “change” and disapproval of Republicans, President Bush and the status quo.

If voters decide — maybe after seeing McCain and Obama in televised debates — that Obama passes the fitness test, my guess is that he could win the election by a hefty margin. So, McCain can’t be blamed for raising as many doubts as he can about Obama.

And, there are plenty to exploit. To the extent that “celebrity” was a substitute for “elitism,” even that may be legitimate, although Paris Hilton and Britney Spears are scarcely symbols of snobbery.

More serious — and also legitimately bashed by McCain — are Obama’s opposition to the Iraq surge and refusal to credit its success, his broken promise on accepting public funding for the general election campaign and his energy and economic policies.

A serious debate is now under way — and, but for Obama, it could be going on face to face — about whether to foster energy independence through “all of the above,” including drilling and nuclear, or mainly through development of alternative fuels and conservation, as Obama proposes.

And another debate is under way over whether the middle class is better off getting tax cuts, rebates and jobs created by government subsidies and directives, as Obama proposes, or by having business create jobs after getting tax cuts.

When Obama talks about the future, he soars. He talks about creating “a new economy for the 21st century — new energy, new jobs, new hope for America’s families.” He calls on the average voter to participate in “the complete transformation of our economy” and acknowledges that sacrifice will be part of it.

The vision is easily attacked as grandiose and unrealistic. But McCain still needs to enunciate a rival vision. Presidential votes are always bets on the future, and McCain needs to give the electorate something to bet on, not just bet against.