In popular memory, Chicago ’68 evokes images of police and demonstrators clashing – and cops swinging nightsticks at anyone who chanced by – in Grant Park and the old Conrad Hilton Hotel, while the Democratic National Convention proceeded apace. But take it from someone who was there (I was an 18-year-old working for Eugene McCarthy’s campaign): The rage inside the convention hall was every bit as great as the anger without.

It wasn’t just the divisions over the Vietnam War and the sense among the anti-war delegates that Robert Kennedy’s assassination had stolen their chance to end the war and transform their party. The clash was more elemental. Just as the yippies and the police fighting on the streets seemed to come from two different Americas, so the party regulars (some representing superannuated machines that had been around since the 1920s) and the earnest young reformers (some representing raggedy, born-yesterday bands of college activists and inner-city agitators) plainly came to loathe each other. I remember standing in the lobby of the Hilton the morning after the convention ended, a bedraggled kid who several hours earlier had been rousted out of the McCarthy junior-staff floor (the 15th, I think), along with the entirety of the McCarthy junior staff, by stick-swinging cops who’d run out of people to club on the street, and telling this sad tale to a delegate, a crewcut, chamber of commerce type who heard me out, looked me over, uttered one word – “Good!” – and stalked away.

Did I mention that the Democrats lost that year?

This year, thankfully, is not 1968. This year, the Democrats are in agreement about leaving Iraq. This year, no great policy differences divide the two front-runners or their supporters. So why do I hear ancestral voices prophesying war?

With neither Hillary Clinton nor Barack Obama yet able to dispatch the other in the primaries, and with the distinct possibility that the primary season could end without a decisive resolution, the possibility of a bitter convention fight, snatching defeat from the jaws of Democratic victory in November, cannot be dismissed.

If one candidate establishes a clear edge over the other in delegates, popular votes and polling against John McCain, I’m certain the superdelegates will swoop down to ensure his or her nomination. If not, the party faces several potentially divisive fights.

The first is over the Florida and Michigan delegations, in which Clinton holds a clear edge and which, by party rules, cannot be seated because their states jumped the gun on the party’s primary calendar. Though the Democratic National Committee announced well in advance of the primaries that no delegates were at stake and that no candidates could campaign in those states, rank-and-file Democrats flocked to the polls, anyway. Now, Clinton insists she won those delegates fair and square, though in Michigan, Obama and John Edwards had obliged the party by removing their names from the ballot, something Clinton strategically neglected to do.

If she thought it could win her the nomination, Clinton could take the question of seating those delegations to the floor of the convention, though the idea of prevailing on the strength of Michigan’s one-candidate primary (OK, two: Dennis Kucinich also left his name on the ballot) would doubtless infuriate Obama’s legions. If the nomination is still up for grabs after the last primary in June, Obama’s campaign – and an array of party leaders as well – will probably propose holding caucuses in Michigan, and maybe Florida, to give those voters an election that counts. The Clinton campaign and the Florida state party, at least, aren’t likely to go along.

And if the election remains unresolved when the primaries end, the Democrats also face a summer problem. Between the last primary, on June 7 in Puerto Rico, and the opening of the convention Aug. 25 in Denver stretch nearly three months during which McCain will campaign as his party’s nominee. Lest the Clinton-Obama battle roll on through those months with no resolution, the Democratic superdelegates need to cast their votes in public and with dispatch after June 7.

There are fault lines – of race, gender and generation – in the party that have opened in the course of the campaign. An insurgency threatens an establishment. And if a battle over contested delegations goes clear to the convention, if the superdelegates anoint a nominee in a process and for a reason that isn’t clear to all, then the fight in Denver, like the fight 40 years ago in Chicago, won’t simply be over an issue or even a nomination. It could well be a furious battle among core Democratic constituencies for the future of the party. And the only winner of that fight would be John McCain.