The central fact of Sen. Barack Obama's candidacy is that no one - whether supporter or detractor - really knows what he's about.

Some Democrats believe he is a progressive, others a centrist. Some believe he will favor a European-style internationalist foreign policy, others a hard-headed American exceptionalism. Republicans worry that he is a paleo-liberal on domestic matters and a naïve accommodationist in foreign affairs.

There is evidence to support all of these suppositions. Whether by accident or design, Obama's record is thin and ambiguous.

For instance, what does Obama really think about taxation? In the course of his campaign, he has promised not to raise income taxes on families making less than $250,000. Yet he proposes lots of other tax increases: on capital gains and dividends, estates and businesses.

Obama proposes "tax cuts" for 95 percent of Americans. But much of that really comes down to tax credits for people who don't pay federal income taxes, of whom there are quite a few.

So who is the real Obama? The sensible, centrist tax-cutter, or the progressive given to "spreading the wealth around"? Perhaps Obama's true beliefs about taxation were revealed in a 2001 radio interview in which he spoke - approvingly, it seemed - about "economic justice" and "redistribution of wealth."

On the subject of Iraq, even Obama's advisers don't quite know what he thinks. During the primaries, he promised to withdraw all combat brigades within 16 months. But adviser Samantha Power explained in March that this was just posturing: "He will, of course, not rely on some plan that he's crafted as a presidential candidate or a U.S. senator." (Power subsequently resigned.)

Another adviser, Colin Kahl, suggested that Obama might leave behind a "strike force" of 60,000 to 80,000 troops - which is a long way from withdrawal. Obama's campaign disavowed Kahl's numbers, but it has committed to the vague notion of some kind of strike force somewhere in the region.

The uncertainty over what Obama would do in Iraq is part of a larger confusion about his foreign-policy worldview. He talks about reclaiming the nation's international good standing, but many of his ideas seem designed to alienate U.S. allies.

For instance, Canada was miffed when Obama, during the primaries, called the North American Free Trade Agreement "devastating" and "a big mistake." (After wrapping up the nomination, he said his criticisms had been "overheated," whatever that means.)

Obama also insists he would be willing to send forces into Pakistan to get Osama bin Laden, even without the country's permission.

Colombia, the most important U.S. ally in South America, is not thrilled that Obama wants to negotiate with Venezuelan President Hugo Chavez, who supports the FARC, a Colombian rebel group.

And France has been unhappy with Obama's unilateral call for negotiations with Iran. French President Nicolas Sarkozy reportedly called the proposal "arrogant."

Which directive would take precedence in an Obama administration: achieving consensus with allies, or engaging adversaries? Who knows? Perhaps Obama would eschew organizing principles and take a pragmatic, case-by-case approach to foreign affairs.

There are questions about Obama's governing style, too. On the stump, he says he wants a more transparent government. His campaign has introduced a host of proposals to promote transparency.

Yet the campaign has sent threatening letters to radio and TV stations in an attempt to bully them out of running anti-Obama ads. It has asked that friends and former classmates not speak to reporters. And it has organized supporters to bombard media outlets hosting unfriendly viewpoints.

So is Obama an idealist or a traditional hardball pol? When he ran for the Illinois state Senate, his insistence on flawless petitions got all three of his opponents removed from the ballot. Yet, recent reports say 30 percent of the 1.3 million new-voter registrations gathered by the liberal organization ACORN - which are likely to favor Obama - were fraudulent. No word on whether he finds this troubling.

Is the real Obama a McGovern progressive? An LBJ-style machine politician? A Reaganite ideologue? A Clintonesque triangulator? Perhaps he's something altogether new. No one, save Obama, really knows. And anyone who claims to know is trying to sell you something.

At this point, voters may not mind that they don't know who Obama is. They know he isn't George W. Bush, and that may be enough.

E-mail Jonathan Last at jlast@phillynews.com.