This is a fast moving situation and any comments can quickly be overtaken by events. Much depends on how bad the results of the elections, and the first indications is that they are bad - as expected - for Labour. It also looks as if minor parties, but especially UKIP, have done well in the European elections.
Will Gordon Brown be the first ever Labour prime minister to be forced out while in office (Blair weakened his position by announcing that he was going sooner or later anyway)? Here are a few observations:
1. Although there is no conspiracy, the actions of James Purnell and others do look a bit like Blairites getting revenge for what happened to the Great Leader.
2. The backbench revolt route was never going to work because it is too convoluted a procedure.
3. Hence, as with Margaret Thatcher, the real blow would have to come from a number of Cabinet ministers. David Miliband is remaining omniously silent. Much will depend on whether Brown has the authority to carry out an effective reshuffle, now said to be occurring today.
4. Having waited so long to get the job, Brown will not go willingly or quietly.
5. Labour should be careful what they wish for. A new prime minister would be under a strong obligation to hold an early general election which Labour would lose. It is difficult to see any scenario under which Labour could win an election because after twelve years in office this is an exhausted party that has run out of ideas. Alternation of parties in government is a key element of democracy.
Incidentally, I don't favour an early election because in the current ferbile atmosphere we wouldn't get a serious discussion of policy options, not least in relation to the economy which remains far more important than MPs' expenses.