Just 12 days to go in Glasgow East and the opinion polls look unforgiving for Gordon Brown's premiership. Alex Salmond, out buying a basket of shopping in Shettleston, detected the beginnings of a seismic event in a YouGov survey that puts the SNP at 33% to Labour's 29% in Westminster voting intentions, the first time this has happened.

"I think the ground is starting to tremble and shudder. I think the earthquake is on the way," said Mr Salmond as he visited Tesco.

Well, a rumbling stomach is a common experience for hungry shoppers. The Labour camp, beset by gallows humour, joke that 29% is actually a good showing but the exterior signs are of extreme nervousness.

The party denies outright that the campaign team asked for or was involved in a move to delay a vote on the controversial Embryology Bill at Westminster to placate some of the Catholic voters in the constituency.

It would have been a crass move designed to appeal to a few hundred voters, but the outcome of the by-election may rest on such small numbers.

Labour's campaign team feel they are back in the ring this weekend after the worst possible start when their favoured candidate went overboard and they had to scramble for a replacement.

Margaret Curran, the Baillieston MSP, has given the local campaign an energising slingshot, but at Westminster there is a gathering mood among Labour MPs that if Glasgow East goes then so too does Gordon Brown.

Fatalists in their ranks think replacing the leader will make no difference and there are black humourists who see victory in Glasgow East as a defeat. Labour historians think this will not be Waterloo for Mr Brown, it will be his Darlington.

In 1983, with Labour in the doldrums, the Darlington by-election was seen by coup plotters as the best chance to dump Michael Foot in favour of Denis Healey.

Labour hung on to the seat, by 2400 votes, the opportunity to change leaders passed and Mr Foot led the party into a massacre at the general election three months later.

But it wasn't just Gordon Brown who felt the heat from Glasgow East last week. Alex Salmond had his "Michael Howard moment" with Jeremy Paxman when the BBC interrogator asked the First Minister repeatedly when he would deliver on election promises made last year.

In Newsnight's London studio, Mr Salmond had made a mistake by insisting upon the convention that party leaders only debate with other party leaders and refused to go head-to-head with Scotland Office Minister David Cairns.

Paxman, faced with two rival politicians who had to pretend the other didn't exist, made Mr Salmond pay the price with the hottest roasting he's had from anyone, politician or journalist, since he took office.

The exchange played out the dichotomy at the heart of the Glasgow East by-election. In London Mr Salmond still regards himself as the opposition maverick out to trip up the government, but in Scotland he is the government.

The arguments in Glasgow East are between incumbent governments in London and Holyrood, each calling the other to account over their respective parliamentary record.

Labour's issues are crime and punishment, claiming that there are 200 fewer police officers in Strathclyde when more were promised, and the SNP is going to keep the pressure on the cost of living under Labour.

Behind the slogans, another election campaign is going on.

When Margaret Curran came out of the Labour selection meeting on Monday night declaring "Labour's fightback starts here" she couldn't have made a more personal statement.

Ms Curran, an MSP since the establishment of the parliament, faces the unappetising prospect of scrambling for a seat in the next Scottish election because population changes will do away with her Ballieston constituency. Win a Westminster seat and she would be free in one bound. Even if she loses Glasgow East but proves herself "a bonnie fechter", as one admiring Labour MP described her, she is all the better placed for the constituency musical chairs and for a bigger prize - the Scottish Labour leadership.