Last week’s vote in Pennsylvania was an even worse result for the Democratic party than is widely supposed. Hillary Clinton’s impressive victory will sustain her campaign through all the remaining presidential primaries, even if Barack Obama bounces back on May 6 in Indiana and North Carolina. At the same time, though, Mr Obama’s campaign did not collapse. Far from it: he made big inroads into the lead that Mrs Clinton once had in the state.

Therein lies the problem. The result in Pennsylvania does not license the party’s “super-delegates” to get behind Mrs Clinton and overrule Mr Obama’s unassailable lead in elected delegates. Pennsylvania was a calamity because it resuscitated Mrs Clinton without coming close to crippling Mr Obama.

Of course, prolonging the ill-tempered battle between Mr Obama and Mrs Clinton hurts the Democrats and helps John McCain, the Republican candidate. For the Democrats, this is bad enough – but it is not the half of it. When this race is over, there will be a loser with ample reason, in either case, to challenge the winner’s legitimacy. The prolonging of the campaign is not the main problem. The greater danger for the Democrats comes at the termination of an exquisitely close race – in a bitterly divisive outcome, whoever prevails, regardless of whether it happens sooner or later.

You can read the rest of this column for the FT here.