IT was a Nightmare On Downing Street, a real Halloween horror.

According to last week's astonishing Scottish opinion polls, the SNP could be in line to return over 40 Scottish MPs at the General Election in seven months' time.

According to Ipsos MORI, the SNP are on 52% - 26% ahead of Labour - their highest rating ever. If this were reflected at the General Election, Labour would return only four out of 59 Scottish seats. A less sensational YouGov poll, conducted after Jim Murphy had thrown his hat in the ring, put the SNP "only" 16% ahead of Labour and in line for 47 seats. We have never seen polls like these in Scottish history.

This means that Alex Salmond, assuming he returns to Westminster on this wave of nationalist support, could hold the balance of power in any UK government. With Britain heading for another hung parliament, even if the SNP returned only half the number of MPs indicated by the polls, they decide whether Ed Miliband or David Cameron secures a Commons majority.

The implications of this are almost too bizarre to contemplate. For one thing, Salmond could theoretically become the deputy prime minister of the UK, in the same way that the Liberal Democrat leader, Nick Clegg, holds that post right now.

Of course, the polls have come at a moment when Labour is leaderless in Scotland and in a state of internal civil war. Many are dismissing the polls as a nationalist "spasm". Nevertheless, the situation faced by the unionist parties in Scotland is potentially very serious.

Even more remarkable than the voting intentions poll was Friday's Ipsos MORI survey suggesting that no fewer than two-thirds of Scots want another independence referendum within 10 years. This isn't just a few nationalist diehards on the internet calling for a re-vote. It suggests widespread discontent in Scotland about how the UK parties have conducted themselves since the referendum.

The attempt by David Cameron to link "English votes for English laws" to devolution for Scotland went down badly, as did the apparent rejection of "devolution max" by the Smith Commission. The appearance of a stitch-up by the unionist parties has not appealed to Scottish voters and nor did the revelation, from Johann Lamont when she resigned as Scottish Labour leader, that "London Labour" had been treating its Scottish party as a "branch office".

And I'm not entirely sure that the sudden appearance of Jim Murphy - the egg man - as the likely leader of the Scottish Labour Party is hugely popular either. He is a Blairite moderniser who supports Trident, the Iraq war and policies like tuition fees, and was considered too right-wing for Miliband's inner cabinet.

The Scottish press are very fond of Murphy, but many voters will recall his antics in the referendum when he made an almighty fuss about being heckled and hit by an egg. He paraded around in his stained shirt as if it were a sucking chest wound and accused the nationalists of an organised campaign of mob intimidation.

Well, it doesn't look as if many voters agreed with him. The SNP's popularity right now reminds me of the wave of almost ecstatic support enjoyed by the UK Labour Party after the 1997 General Election.

The only difference, of course, is that Labour actually won the 1997 election by a landslide whereas the SNP lost the independence referendum badly. By what logic does a party lose on the single issue that defines its political existence and then sweep the country? What message is the Scottish electorate sending here?

I suggested before the referendum that if Scots voted No they might still re-elect the SNP in Holyrood in 2016 if only to keep Westminster from backsliding on its promises. But something else is going on here, something fundamental.

Scots have traditionally split their votes, supporting Labour in general elections to keep the Tories out at UK level, while voting SNP to keep politics alive at Holyrood. Over one million supported Labour in 2010, then gave Salmond a landslide in Holyrood in 2011. Perhaps Scots are now reassessing their historic support for Labour in Westminster.

They are clearly unhappy with the conduct at the referendum of all the UK parties, not just the Conservatives. Many Scottish voters believe, with justification, that Scotland was bullied into voting No by Westminster politicians who appeared to threaten to wreck the Scottish economy.

Voters noted that Miliband not only enthusiastically backed the Tory Chancellor, George Osborne's diktat on currency union, but also said he would put a clause in Labour's election manifesto that Scots would never be allowed to use the pound.

Labour's shadow chancellor, Ed Balls, even suggested he might erect border posts at Gretna to keep immigrants out of England. The Scottish Labour leader, Johann Lamont, shared a platform with the Conservatives on Edinburgh's Calton Hill on the monument called Scotland's Disgrace.

Scots also noticed how, no sooner had Scots safely voted No, than all the talk of "devo max", "home rule" and "near federalism" began to disappear as Gordon Brown, the "saviour", retreated into the background. The Scottish Tory leader, Ruth Davidson, simply announced at the Tory conference that devo max is a "non-starter".

So, we could be seeing an historic realignment in Scottish politics from voters, some of whom voted No, who are not happy about the way Scotland is regarded by the UK parties and press. The idea that Scottish MPs would be determining the future government of the UK is an attractive one. The only problem here, paradoxically, is the SNP itself.

The now-defunct Yes Scotland campaign somehow managed to create a wide non-party movement for independence, which brought in many who were never SNP voters. They may not trust the party not to do a deal with the Tories in any power-broking in a hung UK parliament.

The SNP passed a conference resolution many years ago never to form a coalition with the Conservatives. They would be wise to resurrect it. Also, at present the SNP refrains from voting on English legislation, which could make coalition with Labour difficult, unless on a very limited "confidence and supply" basis.

Finally, many nationalists might find installing Miliband in Number 10 too much to take. What's the point of voting SNP if you get Labour anyway?

To capitalise on this extraordinary home rule movement, the SNP must place it out of its own hands. It must be seen to include non-nationalists, Greens and independents. They should reconstitute the Yes campaign in a different form as a civic Scotland movement, a legitimate descendant of the Scottish Constitutional Convention.

Flying under a home rule flag, the SNP could turn the General Election into a watershed. It could turn UK politics upside down in the way the Bloc Québécois did for Canadian politics in the 1990s. And break Labour's grip on Scotland.