At a time of raging federal budget deficits, one big question emerging from Sen. Barack Obama’s (D-Ill.) expansive agenda is: What are his priorities?

[IMGCAP(1)]And, perhaps an even bigger question is: Is Obama the anti-corporate populist he appears to be on the campaign trail or the “pro-growth” and “pro-market guy” he described in a recent interview with Fortune magazine?

Some hint of answers to these questions may be heard at the Democratic National Convention in Denver. More likely, they’ll have to be teased out in presidential debates this fall.

Democrats legitimately complain that President Bush and Congressional Republicans piled up deficits that raised the national debt from $5 trillion to $9 trillion over the past eight years. This year’s deficit will add close to $500 billion.

But that’s also the inheritance of the next president — along with the prospect that surging health care costs and the retirement of the baby boom generation will explode spending for Medicare and Social Security.

A new report by the nonpartisan Committee for a Responsible Federal Budget — titled “Promises, Promises: A Fiscal Voter Guide to the 2008 Election” — indicates that in 2013, at the end of the new president’s first term, Sen. John McCain’s (R-Ariz.) agenda would add between $288 billion and $354 billion to the deficit and Obama’s, $387 billion.

The committee’s executive director, Maya MacGuineas, acknowledged that these totals were derived from the campaigns’ own estimates and that, as a tax-exempt organization, she is not allowed to declare them “bull or no bull.”

McCain has been rapped often in the media for promising to balance the budget by 2013 while also proposing to extend all of President Bush’s tax cuts at a cost of $294 billion a year and cut other taxes to the tune of nearly $200 billion, while offsetting the costs with unspecified spending cuts, freezes and earmark eliminations of $315 billion.

But, as Obama goes into his nominating convention with a Democratic platform at least as ambitious as his campaign’s own “Blueprint for Change,” the fiscal impact of his plans has received far less scrutiny.

Moreover, as a governing matter, the expansiveness of Obama’s agenda raises the important question: What tasks are going to get which priority. Clearly, removing troops from Iraq will be a major item along with adding troops to Afghanistan.

And, clearly, if the economy is still weak in January, Obama and the Democratic Congress will want to enact a stimulus package. Obama recommends $50 billion — including new rebates to middle-class households and aid to cities and states — but Congress easily could increase that number.

After that, however, what would the Obama administration tackle? The candidate has said he wants to enact universal health care reform by the end of his first term and he’s also said that transforming the U.S. economy from its dependence on imported oil is “the greatest test of our time.”

Both of those objectives involve complex, expensive and politically controversial steps. Obama’s health proposal involves heavy regulation of the health insurance industry, a $50 billion investment in health care technology and at least $65 billion a year to give the 47 million uninsured the same coverage as Members of Congress have, plus creation of a government-run health plan to compete with private industry.

On the energy front, Obama — like McCain — favors a cap-and-trade system to control global warming, but Obama is proposing nothing less than “a complete transformation of our economy” that will be “costly and will require us to defer some other priorities.”

Obama wants to invest $150 billion to create alternative energy sources. He’ll raise fuel standards for cars and require use of renewable fuels by utilities and ask all Americans to cut electricity demand by 15 percent. He acknowledges the transformation will be “costly,” but there is no estimate of its short-run effects on GDP or wages.

He has also proposed massive changes in tax legislation — extending all the middle-class tax cuts of the Bush era, which are scheduled to expire in 2010, while allowing those to lapse for filers earning more than $250,000. There will be a college tax credit, a refundable mortgage credit and a “make work pay” tax credit. The tax code will not become simpler.

And on top of all that, Obama wants to increase education spending by $18 billion a year, spend $6 billion more on infrastructure (an amount Congress will want to increase), $20 billion to help homeowners avoid default, $15 billion for basic research, double foreign aid and increase the size of the military.

On the stump, Obama tends to blame corporations, their lobbyists and the Bush administration for what has gone wrong with the American economy, and his health and energy programs depend on heavy government regulation. Moreover, he wants to give labor unions all they ask for in terms of organizing power, minimum-wage increases, limits on free trade and prevailing wage rates on government projects.

At the same time, he told Fortune’s Nina Easton that he believes in free trade and will cut some corporate taxes. He has some notable economic moderates in his brain trust, including super-investor Warren Buffett, former Fed Chairman Paul Volcker and Jason Furman, former director of the free-market Hamilton Project at the Brookings Institution.

So, when Obama and McCain finally debate this fall, a good question for the Democrat will be: “Of all the things you propose, what do you want to do first?” And another is: “Sen. McCain acknowledges he’s a conservative. Are you a liberal?”