The most telling moment in last night's debate came when Gordon Brown conceded:
I know that if things stay as they are, perhaps in eight days' time David Cameron, perhaps supported by Nick Clegg, would be in office.
This was probably intended to alarm voters (the man who would wreck the economy and slash public services could soon be prime minister), but it sounded like an admission of defeat.
It was also a remarkably ill-judged statement. Like Rupert Murdoch, voters like to back a winner. By admitting that Cameron is set to enter Downing Street on 7 May, Brown has made it all the more likely that he will.
Few voters pay close attention to the opinion polls, but the idea that Cameron is leading a government-in-waiting was in effect endorsed by Brown.
Even the most amateur politician knows to stick to the line that: "There's only one poll that counts, and that's on election day." Brown's decision to break with form and prejudge the outcome of the election must have had Labour strategists hanging their heads in despair.
It's far from unthinkable that Labour could rally and, due to the vagaries of the voting system, emerge as the single largest party. But last night Brown made that outcome all the more unlikely.