Broadly speaking, the serious contenders for the Democratic nomination are offering similar policy proposals - the dispute over health care mandates notwithstanding. But there are large differences among the candidates in their beliefs about what it will take to turn a progressive agenda into reality.
At one extreme, Barack Obama insists that the problem with America is that our politics are so "bitter and partisan," and insists that he can get things done by ushering in a "different kind of politics."
At the opposite extreme, John Edwards blames the power of the wealthy and corporate interests for our problems, and says, in effect, that America needs another F.D.R. - a polarizing figure, the object of much hatred from the right, who nonetheless succeeded in making big changes.
Over the last few days Mr. Obama and Mr. Edwards have been conducting a long-range argument over health care that gets right to this issue. And I have to say that Mr. Obama comes off looking, well, naíve.
The argument began during the Democratic debate, when the moderator - Carolyn Washburn, the editor of The Des Moines Register - suggested that Mr. Edwards shouldn't be so harsh on the wealthy and special interests, because "the same groups are often responsible for getting things done in Washington."
Mr. Edwards replied, "Some people argue that we're going to sit at a table with these people and they're going to voluntarily give their power away. I think it is a complete fantasy; it will never happen."
This was pretty clearly a swipe at Mr. Obama, who has repeatedly said that health reform should be negotiated at a "big table" that would include insurance companies and drug companies.
On Saturday Mr. Obama responded, this time criticizing Mr. Edwards by name. He declared that "We want to reduce the power of drug companies and insurance companies and so forth, but the notion that they will have no say-so at all in anything is just not realistic."
Hmm. Do Obama supporters who celebrate his hoped-for ability to bring us together realize that "us" includes the insurance and drug lobbies?
O.K., more seriously, it's actually Mr. Obama who's being unrealistic here, believing that the insurance and drug industries - which are, in large part, the cause of our health care problems - will be willing to play a constructive role in health reform. The fact is that there's no way to reduce the gross wastefulness of our health system without also reducing the profits of the industries that generate the waste.
As a result, drug and insurance companies - backed by the conservative movement as a whole - will be implacably opposed to any significant reforms. And what would Mr. Obama do then? "I'll get on television and say Harry and Louise are lying," he says. I'm sure the lobbyists are terrified.
As health care goes, so goes the rest of the progressive agenda. Anyone who thinks that the next president can achieve real change without bitter confrontation is living in a fantasy world.
Which brings me to a big worry about Mr. Obama: in an important sense, he has in effect become the anti-change candidate.
There's a strong populist tide running in America right now. For example, a recent Democracy Corps survey of voter discontent found that the most commonly chosen phrase explaining what's wrong with the country was "Big businesses get whatever they want in Washington."
And there's every reason to believe that the Democrats can win big next year if they run with that populist tide. The latest evidence came from focus groups run by both Fox News and CNN during last week's Democratic debate: both declared Mr. Edwards the clear winner.
But the news media recoil from populist appeals. The Des Moines Register, which endorsed Mr. Edwards in 2004, rejected him this time on the grounds that his "harsh anti-corporate rhetoric would make it difficult to work with the business community to forge change."
And while The Register endorsed Hillary Clinton, the prime beneficiary of media distaste for populism has clearly been Mr. Obama, with his message of reconciliation. According to a recent survey by the Project for Excellence in Journalism, Mr. Obama's coverage has been far more favorable than that of any other candidate.
So what happens if Mr. Obama is the nominee?
He will probably win - but not as big as a candidate who ran on a more populist platform. Let's be blunt: pundits who say that what voters really want is a candidate who makes them feel good, that they want an end to harsh partisanship, are projecting their own desires onto the public.
And nothing Mr. Obama has said suggests that he appreciates the bitterness of the battles he will have to fight if he does become president, and tries to get anything done.
Paul Krugman is Professor of Economics at Princeton University and a regular New York Times columnist. His most recent book is The Conscience of a Liberal.
Copyright 2007 The New York Times Company