In "The Exterminating Angel," a 1962 film by Luis Bunuel, the great Spanish anarcho-surrealist director, the guests at a dinner party find that, mysteriously, they cannot leave. Though there are no external constraints to their exiting, none can cross the threshold of the music room to which they've adjourned. For days and days they stay, some growing to hate one another, some lapsing into despair and most eventually determining to sacrifice their host in the hope that killing him will set them free. (They manage to get out before the host has been dispatched.)

Democratic voters awoke Wednesday to find themselves living out a primary season alarmingly like Bunuel's mordant fantasy. Nobody wished for a process that would roll on through summer or envisioned a contest in which the party's constituencies were arrayed against each other, in nearly equal force and with only minor variations, in state after state after state. Nobody anticipated that two candidates with no great policy differences would battle it out to no readily apparent resolution. Yet that's exactly what has happened. The Democrats are stuck.

Barack Obama and Hillary Clinton both have every reason to keep on campaigning, but the remaining contests will probably settle nothing. She almost certainly can't erase his lead among pledged delegates, and he hasn't demonstrated yet that he can win enough white working-class voters in key states to convince an overwhelming majority of superdelegates that he should be their pick. Clinton and Obama have divided up the Democrats' political world along distinct constituency lines, and only in a handful of states has one of them won over the other's demographic base — chiefly Wisconsin, where Obama managed to win less-affluent white voters.

Indeed, this year's contest is proceeding much like a political-demographic census, in which the class, race and age breakdown of each state's electorate gives you a pretty fair idea of who's going to win. Handicapping which candidate would run better against John McCain is no easy task, either. Obama can clearly attract more upscale independents than Clinton can, and her claim on less-affluent whites who shifted to the Democratic column in the 2006 midterms looks stronger than his. He'd probably do better in the Mountain West; she might do better in the Rustbelt Midwest.

Eventually, of course, the Democrats will have a nominee — but how to determine whom without wrecking the party's prospects in November will require sound judgment and firm leadership from Clinton, Obama, party Chairman Howard Dean and other party leaders. First, the party needs to schedule primaries in Florida and Michigan — preferably in June, soon after Puerto Rico, so that it doesn't add a crisis of legitimacy to its accumulating difficulties. Second, party leaders must make clear to the candidates that some attacks and innuendos should be out of bounds — such as Clinton's hemming and hawing on "60 Minutes" over whether Obama really is Christian. That caution should be conveyed privately, but if such ploys continue, then Dean, House Speaker Nancy Pelosi and the as-yet-unheard-from Al Gore should condemn them publicly.

Also, somebody is going to have to focus on McCain, who will merrily be depicting the Democrats, particularly Obama (Clinton is a known quantity), in the worst possible light. If the Democrats are to carry Ohio, Pennsylvania and Michigan in the general election, they need to put a spotlight on McCain's continuing infatuation with trade policies that have taken the Midwest's good jobs offshore. Obama and Clinton will attack McCain on trade while stumping in Pennsylvania, but their chief focus is more likely to be on attacking each other.

McCain's vulnerabilities as a candidate are legion. He accepted his party's de facto nomination Tuesday night without mentioning any ideas on how to get us out of the coming recession. His perfervid support for the Iraq war has blinded him to the bigger strategic picture, which is that the predictable pro-Iranian tilt of Iraq's post-Saddam Hussein leaders has helped Iran in its drive to become a regional superpower. Now, McCain speaks of war against Iran to undo the consequences of the war in Iraq that he has championed. But who will be broadcasting a critical look at McCain's record while Clinton and Obama duke it out? The Democrats' biggest donors — unions from both sides of the candidate divide and business executives who funded the independent expenditure campaigns of 2004 — will have to assume this task in the absence of a presumptive nominee.

Because just now the Democrats can't find their way out of a primary contest that almost surreally refuses to end.

Harold Meyerson is editor-at-large of American Prospect and the L.A. Weekly. He wrote this commentary for The Washington Post.