He'll need to raise the support of 20% of MPs in order to do so.
I've compiled a list of who it could be (fitting the description of former Cabinet Minister):
- Charles Clarke (possible, a staunch critic of Brown)
- Ruth Kelly (resigned not long ago - somehow I doubt it)
- John Reid (possible, but said he was resigning at the next GE, so unlikely)
- Patricia Hewitt (seems unlikely)
- Peter Hain (possible, but disgraced over donations fiasco in Deputy Leadership race)
- David Blunkett (possible, a staunch Blairite)
- Hilary Armstrong (seems to be a Blairite, possible)
- Frank Dobson (anti-Blair, possible)
- John Prescott (seems unlikely having resigned due to an affair - now busy with the election campaign called Go Fourth)
- Michael Meacher (challenged Brown during first contest - seems unlikely to do so again)
- Alan Milburn (was suggested by Clarke as a contender the first time - very possible)
- Andrew Smith (could be - only one to have voted against the Government over Gurkhas)
- Stephen Byers (criticises Brown - very possible, but very New Labour)
- Alun Michael (possible)
From that list, the most likely appear to be Charles Clarke, David Blunkett, Hilary Armstrong, Frank Dobson, Alan Milburn, Stephen Byers, Andrew Smith.
Well, we'll see, and it depends on the government losing the Expenses vote.
[UPDATE]: well, stuff all that - the government won the expenses votes, BUT the possible contender will still be out there, waiting for their chance!