By James Landale
Chief political correspondent, BBC News Channel
What is likely to be the fall-out from the Conservative Party's first by-election gain from Labour in 30 years?
Labour's class war campaign tactics backfired in Crewe and Nantwich
Gordon Brown's future as prime minister now hangs on two things - the response of Labour MPs to the party's historic and overwhelming defeat in Crewe and Nantwich and his own analysis of why thousands of former Labour voters in this Cheshire constituency gave their support to the Conservatives.
Some Labour MPs will panic, perhaps even going so far as to call for Mr Brown's head in public.
Most though think a move against Mr Brown is unlikely in the short term, not least because there is no obvious successor and any leadership contest would be divisive and damaging.
But the size of the Tory majority in Crewe and the scale of the swing against Labour will prompt some MPs to wonder if their jobs might be safer under a new leader.
People are already talking of things coming to a head at the autumn party conference if things do not get better.
So much for the MPs, now for the post-match analysis.
First the campaign. It was poor and clumsy. The class war tactics against the so-called Tory "toff" candidate backfired, portraying Labour as negative and anti-aspirational.
Mr Brown did not campaign in Crewe and Nantwich
Nor did the choice of Gwyneth Dunwoody's daughter, Tamsin, as Labour's candidate, pay off. In the end she won more hugs than votes.
Yet a poor campaign did not lose this seat for Labour.
The real causes for Labour's defeat were the economy and voters' doubts about Gordon Brown.
Prices are rising, people in Crewe are feeling the pinch, but they appear unsure what the government was doing about it.
They disliked paying more tax as a result of the 10p rate abolition, and the compensation package came too late to have an impact.
Voters also used the '10p issue' as a proxy to voice their concerns about Mr Brown. He did not campaign, his name did not appear on Labour leaflets.
But the Tory slogan urging voters to "send a message to Gordon Brown" clearly resonated.
So what might Mr Brown do?
On the economy the prime minister is expected to look at more short term help to support those suffering most from the economic downturn.
That will need cash, which he does not have, so long term investment budgets in the public services might be plundered while he waits for the economy to improve.
As for the man himself? He is clearly not going to go voluntarily, a putsch is unlikely and all attempts at another makeover look implausible.
But the aim somehow would be to restore Mr Brown's reputation as a safe, competent, prudent ex-chancellor who can steer us through the slowdown.
As for the Tories, does their victory show they are on course to win the next general election?
No. By-election results are notoriously poor predictors of national poll outcomes. But by-elections, particularly historically important ones like this, can have a huge impact.
The result will boost Conservative party morale. It will encourage financial donors, but above all it will make the Tories appear more clearly in the minds of voters as a potential future government.
In other words there is a virtuous circle here of success breeding success.