Maybe the biggest question of the 2008 presidential campaign is “Who is Sen. Barack Obama really?” Of late, the mystery is deepening.

[IMGCAP(1)]It’s customary for presidential candidates to move to the center for the general election after they’ve pandered to their party’s base in the primaries — but the Illinois Democrat has claimed not to be your customary candidate, but someone who was going to usher in a new politics.

He has eloquently promised “change we can believe in,” but lately he’s changing his tune on so many issues it’s becoming a legitimate question: Can voters really believe in him?

In a year when Democrats have everything going for them, I think the primary reason Republican Sen. John McCain (Ariz.) is trailing Obama by only 2 points in the Gallup Poll is that voters still have their doubts about Obama.

Some of the doubts are ridiculous or even pernicious — such as whether Obama is a patriotic American, a Christian, a person who “shares your values.”

He brought some of this on himself — by saying that wearing an American flag lapel pin constituted “false patriotism” and by remaining for 20 years in the racially incendiary Rev. Jeremiah Wright’s church.

Some of the rest of it — such as that Barack Hussein Obama is really a Muslim — is being spread on the Internet by haters.

It’s perfectly reasonable that Obama take steps to make it clear that he really does love and appreciate the country and that he shares middle-class values. So, he’s put the pin back on. In TV images as he speaks, he’s practically wrapped in American flags. He’s re-explained his ancestry and his upbringing.

He has a problem with white working-class voters, and if he takes his position to appeal to them — or changes positions — it’s probably no big deal.

Obama also endorsed aspects of President Bush’s faith-based initiative. White working-class voters do love their guns, their churches and law and order, after all.

It also undoubtedly helps Obama with the crucial independent vote for him to cause dismay among his supporters on the MoveOn.org left — as he did by dropping his pledge to filibuster the grant of legal immunity to telecommunications companies taking part in Bush’s terrorist surveillance program.

I happen to think it was the right decision on the merits — and the Foreign Intelligence Surveillance Act reform bill needed to be passed promptly, so Obama was bowing to reality.

But much more dubious — in fact, raising questions of character — is his abandonment of a solemn promise to run his general election campaign with public funds if his Republican opponent did.

Obama still says he favors public financing and he even claimed with a straight face that his collection of hundreds of millions of dollars from small, private donors — along with a lot from big donors, too — actually constitutes public financing.

From this episode — which probably matters only to political insiders — we learn that Obama is a politician of … shall we say, flexible principles. Pastor Wright told us as much before Obama disowned him, although the pastor certainly deserved to be disowned.

Then there’s his position on the North American Free Trade Agreement, which he said during the primaries — and continues to say on his campaign Web site — should be renegotiated. But lately he’s said that statement was “overheated.”

It seems he wants it both ways — satisfying the anti-free-trade AFL-CIO on the one hand and a passel of pro-trade New York investment bankers on the other.

And, finally, there’s Iraq. It’s been a bedrock principle of his campaign that he would pull all U.S. combat troops out within 16 months of taking office. Last week, in one press conference, he said he might “refine” that schedule.

In a hastily called second press conference, he said, no — he was sticking by the schedule, though the pace of withdrawals might change from month to month, depending on circumstances and military advice.

Is this a flip-flop? A canny pragmatist’s bowing to changed circumstances and the dawning awareness that U.S. foreign policy might well be on his shoulders?

It would convince me that he was a daring man of character if he went to Iraq, saw Gen. David Petraeus and the situation on the ground and came back saying: “This war was wrong at the start, but now we have to win it — and we can win it, politically and militarily. We will withdraw — but only under conditions of success.”

Such a statement would finally show that he can buck the dominant attitude of the Democratic Party. If he added that he was wrong to oppose Bush’s 2007 troop surge, so much the better.

Pending such an unlikely event, the question is open: Is this guy the real deal, or an eloquent phony? A flip-flopper, a cynic, just an ordinary pol with a gift of the gab — or a genuine center-liberal capable of tacking while steering a determined course?

There’s time to find out before November, but the media have to help with intense, ongoing scrutiny and lots of tough questions.