WASHINGTON – The presidential candidates of both parties have been campaigning for months now, introducing themselves to the nation. So why do so many of them seem to get progressively fuzzier and less distinct, like photographs left out in the sun? Is it the process that’s causing this steady attenuation, or does the problem lie with the candidates themselves?

There are two major exceptions to the general rule I’ve just posited. Hillary Clinton and John McCain are as vivid as the subjects of the artist Chuck Close’s hyperrealist portraits, in which you can see every wrinkle, every blotch, every pore. The problem is that when people know exactly who you are, or think they do, they tend to form hard and fast opinions. There are so many Democrats who “just don’t like Hillary Clinton” and so many Republicans who “just don’t like John McCain” that the two candidates once considered presumptive favorites to win the major-party nominations could both fall short.

By contrast, the images of their opponents range from gauzy to blurry to practically incorporeal.

Barack Obama’s phenomenal rise as a candidate came as he was brilliantly sketching the outlines of who he is and what he believes. His identity and philosophy are based on inclusiveness, which is a soothing message for a nation bleeding from wedge-issue politics as practiced by George W. Bush, Karl Rove and the like. But not all either/or propositions are false choices – some are real choices, with real consequences. While we know much about who Obama is and how he thinks, the question of precisely what he would do in a given situation is like the bottom line on an eye chart – you can almost make it out, but not quite.

Obama’s sudden prominence lowered a scrim in front of John Edwards’ candidacy – he was going to be the anti-Hillary, but that role is now taken. Joe Biden certainly speaks his mind clearly enough, but it has been hard to persuade people to see him and Chris Dodd as anything other than creatures of the Senate – unlike Clinton and Obama, two senators who are blessed with escape-velocity star power.

We thought we knew Rudy Giuliani, who started out as an icon: “America’s mayor,” the hero of 9/11. But then he began shape-shifting. New Yorkers were familiar with a mayor who was vocally pro-choice on abortion and strongly in favor of gay rights. Republican primary voters are being introduced to a Giuliani who is reluctantly pro-choice, with a heavy heart, and who doesn’t really want to talk about gay rights much at all.

He should take a lesson from Mitt Romney, who at least looks and sounds convincing as a committed conservative – even though he managed to get elected governor of Massachusetts, pretty nearly the least conservative state in the nation. Romney doesn’t try to pretend he hasn’t changed his positions, he just comes up with a reason for the about-face and sticks to his story.

And as for the rest of the Republicans, I could pick Sam Brownback, Mike Huckabee, Duncan Hunter, Jim Gilmore, Tommy Thompson, Tom Tancredo and Ron Paul out of a lineup, but I’d bet most voters couldn’t.

Maybe it’s the sheer number of candidates that makes them so hard to get a fix on – a “debate” with eight or 10 participants is really no more than a glorified panel discussion. Maybe it’s the extraordinary length of the campaign, which is now about to downshift into summer mode. Some of the candidates who trail in the polls may start running out of money and be forced to drop out between now and September. For the front-runners, all of whom have money to burn, the next few months are mostly about positioning themselves for the stretch run.

Clinton and McCain have settled political identities, for better or worse. It’s not that they have always been consistent – Clinton has taken just about all possible positions on the Iraq War – but people think they know who these veterans are. By the fall, another familiar Washington hand, Fred Thompson, who happens to be a professional communicator, may well be a major player in the Republican field. Newt Gingrich, a known (if mercurial) quantity, may be in there, too.

And by summer’s end, Al Gore will have finished his book tour. You did notice that he has a new book out, right?

Eugene Robinson is a Washington Post columnist. Contact him by writing to eugenerobinson@washpost.com.