Can anyone stop the Sturgeonater's landslide?

On the very day Jim Murphy launched Labour's Scottish election manifesto on Friday, Lord Ashcroft's latests round of constituency polls indicated that he will probably lose his East Renfrewshire seat.

Murphy could be forgiven for thinking it's personal. Lord Ashcroft's polls seem to have been timed to damage Labour every time he tries to get its Scottish campaign on the road. It's like having the Grim Reaper as road manager.

Ashcroft's numbers also crushed the hopes of Murphy's erstwhile Labour rival, Douglas Alexander, the Shadow Foreign Secretary, in Paisley. These were on epic swings to the SNP of up to 33%, completely unprecedented in general elections.

Meanwhile, Nicola Sturgeon was widely held to have been the best performer in the challengers' debate on Thursday, even in the UK-wide Survation poll.

With David Cameron having absented himself, it was very much the Ed and Nicola show. They danced around the issue of whether or not there could be co-operation between Labour and the SNP after the General Election

OK, "danced" may be putting it a little too elegantly. There were some pretty sharp blows exchanged. Miliband tried to dismiss the SNP as irrelevant, erstwhile Tory collaborators, while Nicola accused Ed of preferring a Tory government to working with the SNP.

It was a memorable debate, not least for Nigel Farage's suicidal attack on the studio audience. But we have had so many debates now that the nation is beginning to get argument fatigue. There's only so many we can take.

Indeed, I had a distinct feeling last week that this election campaign has already reached a natural conclusion now that the main arguments - already well trailed - have been laid out in the UK party manifestos. The nation has more or less made up its mind and it is now ready to vote.

Yet there are still nearly three weeks to go.

I suspect that is pretty much how David Cameron planned it. He pulled the debate dates forward to Easter in order to give time for the novelty value of the new faces to fade.

The main problem for Nicola Sturgeon may now be over-exposure. She is regarded a little like a national champion at the moment - a kind of political Chris Hoy. Didn't she do well? Go us. Even non-nationalists confess to being a little infatuated with the First Minister.

But as this column has argued, political love affairs are fleeting and voters are fickle. Remember how Clegg-mania gripped the nation in the first week of the 2010 general election campaign only to evaporate by polling day as voters reflected on his competence to rule the country.

Of course, Nicola Sturgeon is already ruling as First Minister of Scotland and has a hard-earned reputation for ministerial competence. I think this explains some of her composure and confidence in the debates, which has so impressed the Westminster political establishment.

There had been an assumption that she is a political outsider, a kind of nationalist Nigel Farage, and would struggle under the pressure of a general election campaign. But the "wee lassie" as a Scottish Labour MP unwisely called her, has shown that she can more than hold her own.

Jim Murphy can only hope now that there will be a late swing back to the big Westminster parties as polling day approaches. Scots will, he hopes, realise that it is a two-horse race between Ed Miliband and David Cameron. Every vote for the SNP is a vote for the Tories.

Will people buy it? Well, they haven't up to now. The momentum in this election is almost entirely with the SNP. Their lead in the opinion polls is reaching ever-greater heights of unreality - over 52% of the Scottish electorate say they are minded to vote SNP. This doesn't so much look like an election as a revolution.

It's possible that an outbreak of nationalist triumphalism could scare mainly middle-class voters into a round of tactical voting against the SNP. After all, unionist voters had a 55% majority in the referendum.

But the SNP have been highly disciplined so far, even if their raucous supporters on the internet still resort to intemperate language. But even that has become a bit of a joke - literally.

On Friday, after the Ashcroft polls, SNP supporters started tweeting about spontaneous riots taking place across Scotland. "It's Armageddon in Cambusnethan ... Barricades in Inversnaid." By late afternoon, #scotriots was trending across UK-wide. It was a daft collective parody of the intemperate coverage of nationalist politics in the mainstream press.

Cybernats aside, the main negatives for Nicola Sturgeon remain the economics of full fiscal autonomy, and the "£7.6bn deficit" identified by the Institute for Fiscal Studies. But the problem here for Labour is that it's not easy to arouse strong passions over something which isn't going to happen.

There is simply no prospect of fiscal autonomy being introduced in Scotland in the foreseeable future because both Labour and the Conservatives have made clear they will oppose it. Devolution of all taxation would require a vote in Westminster.

Now, this doesn't mean that it is acceptable for a political party to have a headline policy in which the numbers don't add up. But this argument is essentially a repeat of last year's warning about the economics of independence. And the point about the referendum was that 45% of Scottish voters still said Yes.

The SNP only needs to hold on to this 45% to deliver a landslide on May. This is because of our unfair first-past-the-post system of voting, which massively inflates the majority of the winning party. The SNP could get 85% of the seats on less than 50% of the popular vote.

And Jim Murphy is not without fiscal headaches himself. Last week he was very publicly slapped down by his own shadow chancellor, Ed Balls, for suggesting, on the now infamous "stair heid rammy" on BBC Scotland's Sunday Politics that there need be no further spending cuts after 2016. Balls made clear that there certainly will be.

There is a problem also with Murphy's promise to keep the Barnett Formula on public spending "today, tomorrow and forever". Under the Smith reforms - which the Tories and Labour will vote for - the Barnett Formula is effectively being phased out. By 2020 it will apply to barely 30% of the expenditure of the Scottish Parliament. The rest will be raised directly. And even this minimal Barnett will almost certainly be reformed with English devolution.

The days when the Scottish Parliament got a big lump of cash every year, based on a fixed formula of spending in the UK, are gone. This means that there is no fiscal status quo for unionists to appeal to. Whatever happens, Holyrood is going to have to raise a lot more of its expenditure through direct taxation.

Normally, a split with Labour in Westminster, combined with the SNP's crushing poll figures, would have been enough to destroy Jim Murphy's campaign before it got off the blocks. But no-one - in the press at any rate - is writing him off yet. There is always the hope that he might suddenly work a miracle.

But I fear that most Scots voters seem to have made up their minds about Jim Murphy and it is not positive. Wile E Coyote - to whom Jim Murphy is often compared - is finally about to run out of road.