Poll defeat will force quick exit for Brown
Harman tipped as party 'caretaker' for the lengthy leadership battle
Gordon Brown is expected to step down quickly if Labour suffers a decisive defeat on Thursday -- plunging his party into its first leadership contest for 16 years.
Labour insiders say the prime minister could be replaced by Harriet Harman, the deputy leader, as a "caretaker" while Mr Brown's successor is chosen -- in a battle likely to last for months.
A group known as the "ultras", which includes Peter Mandelson, the Business Secretary and the man running Labour's much-criticised campaign, is keen to install David Miliband, the Foreign Secretary, as leader without a formal contest.
But this is unlikely to happen. Any move by Mr Miliband is almost certain to be challenged by Ed Balls, the Schools Secretary, and probably by a candidate representing the left of the party, most likely to be Jon Cruddas, the former challenger for the deputy leadership.
Ed Miliband, the Climate Change Secretary who is the favourite of many Labour MPs, faces an agonising decision over whether to stand against his brother David.
Last week, Labour morale suffered a blow after what one insider described as "36 hours of hell". On Wednesday, Mr Brown was caught calling Gillian Duffy, a pensioner and lifelong Labour voter, a "bigoted woman" during a trip to Rochdale, while on Thursday he failed to make a breakthrough in the final leaders' debate on the economy.
One senior Labour MP said "the life appeared to drain away" from Mr Brown after his Rochdale gaffe. His unguarded comments to a senior aide were picked up by a radio microphone he failed to remove after his altercation with Mrs Duffy, who questioned him on the government deficit and immigration.
"During the debate on Thursday night you could see the experiences of the previous 36 hours etched on Gordon's face," the MP said. "He looked like he had been through the wringer -- and he had."
Mr Brown is said to have "been beating himself up" over the incident with Mrs Duffy and what he perceives as his failure to make any headway against David Cameron or Nick Clegg during the three debates.
And on Friday night, two national newspapers that had followed Labour in the past abandoned the party, one backing the Liberal Democrats and the other, the Conservatives.
It is thought likely that Mr Brown will make a quick departure if the Tories are the clear election winners -- either with an outright Commons majority or with enough seats to form a government.
A Labour insider said: "I cannot see Gordon coming back to the Commons in a few weeks to take on Cameron as leader of the opposition. It would be too cruel."
In such an event, Mr Mandelson is understood to want to arrange a "coronation" for Mr Miliband to avoid a "divisive" leadership contest which would hit Labour financially.
Most Labour MPs are thought to want a contest fought under the auspices of a caretaker leader.
Ms Harman is thought best placed to take the caretaker role, although Alistair Darling, the chancellor, is another possible choice because his non-confrontational style appeals to many Labour MPs.
As caretaker, Ms Harman is thought unlikely to stand for the leadership herself.
Mr Mandelson is believed to fear any intervention by Ms Harman which seeks to tie him to the failure of Labour's campaign. His allies think Ms Harman may take such a step on election night if exit polls predict a bad Labour defeat.