WHAT is he waiting for?

The cry for Ed Miliband to rule out a coalition with the SNP is being heard louder by the day, and not just in the Tory press. Initially it was Scottish Labour MPs like Ian Davidson and anti-Nationalists like Lord Foulkes who were demanding that Ed give Nicola the bullet. But last week more moderate figures such as the respected former health secretary, Alan Johnson, started adding their names to the list calling for a lock-out of the SNP.

And the list can only grow because is about all Labour politicians are asked about now on BBC programmes is: "Will you or won't you rule out a deal with Alex Salmond." (Even though Salmond is no longer leader of the SNP.)

It's become what advertising types call a "meme". From Steve Bell cartoons in the Guardian to the Conservatives' Photoshop of Ed Miliband in Alex Salmond's pocket, the image has stuck. How dare these Nationalists come down here trying to run England?

I doubt if Ed can hold out against it much longer. He'll probably make a formal announcement shortly that Labour is not prepared to enter into any Westminster coalition with the SNP while it opposes Trident and wants to break up Britain.

But will that be enough? Voters are becoming better informed about coalition politics and understand that there are many ways of being in one without formally declaring it.

Labour and the SNP could have an informal deal like the Lib-Lab pact of the late 1970s. They could do "confidence and supply", supporting Ed Miliband's budget and any confidence motions. Or the SNP could simply agree to vote down any Conservative Queen's Speech in exchange for some policy concessions like devolution of broadcasting.

This of course only applies if the SNP do return the 40-50 MPs indicated by the opinion polls and hold the balance of power in the House of Commons. And that's far from certain. But short of that, there are further coalition options involving third parties.

For some reason, no-one seems to talk about a Labour-SNP-Liberal Democrat coalition, but that is certainly a possibility. The LibDems are closer to Labour and the SNP than they are to the Tories on issues like austerity, EU membership, Lords reform, living wage and taxation.

And thanks to the 2011 Fixed Term Parliament Act, it is now more difficult for prime ministers to call a snap election if they lose votes in the House of Commons. They have to win a two-thirds majority among MPs before another election can be called. Which means all the coalition options in Westminster are likely to be explored much more exhaustively than ever before. We are in a new era.

So, just ruling out a formal coalition with the SNP may not be enough. BBC interviewers will continue to ask the Labour leader if he will rule out any and all electoral co-operation with the SNP; much as the SNP have said they will do nothing to help the Tories get into office. That's what Ed Miliband's Scottish MPs want him to say about the SNP.

They want Ed Miliband to give the Scots an offer they cannot refuse: vote for us or see David Cameron back in Westminster. It is a bit like the currency issue in the referendum campaign. If you take away any hope of Scotland using the pound, Scots had no choice but to vote No. (Except many voted Yes, but that's another story.)

Labour and the SNP are social democratic parties of the left with many policies in common, such as increasing free child care and halting Tory welfare reforms. As this column argued last week, a coalition programme could be put together tomorrow. But politics is essentially tribal. Delivering policies that benefit their voters often comes a poor second to party advantage.

It is a counsel of despair, perhaps, but Labour has to find some way to halt the Sturgeon steam-roller. The attempt to outflank the SNP on tuition fees, fracking, income tax and NHS spending doesn't seem to be working. According to the latest Times/YouGov poll last week, Jim Murphy is actually getting more unpopular the longer he is leader.

More than half of Scots, 51% think the Scottish Labour leader is doing a bad job, up from 43% in their last survey. Meanwhile 62% of Scots think Nicola Sturgeon is doing a good job. The SNP lead over Labour is only slightly down at "only" 19%, which would still give the SNP 48 seats in May.

However, You Gov also record that 88% of SNP switchers want Ed to leave open the possibility of a coalition. And that 37% of all voters now believe Labour's claim that voting SNP is more likely to put the Tories back in than voting Labour.

So, what is Ed waiting for? Why not just say No? But it remains a tough call, and Mr Miliband is right to be cautious. Refusing to deal with the SNP might appeal to the Daily Mail, but the SNP will say: "See: told you so. Labour would rather see the Tories in office than form a government with us to introduce social democratic policies. The Tories and Labour are just the same."

Also, and worryingly for Labour, YouGov's survey indicates that Scots actually like Ed Miliband even less than David Cameron. It seems hard to believe that Red Ed could be less popular than the Eton-and-Bullingdon Tory, but the polls have indicated this for some time.

So, there is a risk that Labour's bluff might be called. Scottish voters might well conclude: stuff it, if it's a choice of two unacceptable alternatives, I might as well vote with my gut this time and back the SNP. At least it sends a clear message to Westminster. Especially since they don't seem to want us there.

The SNP is increasingly talked of in Westminster circles, and in the UK press, as if it were in some way an illegitimate interloper in UK affairs. Commentators like Max Hastings talk about how it would be "unacceptable" for 60 million English people to be "ruled by five million Scots". Such chatter is hardly likely to make Scots keen to identify with the Westminster system, which many believe anyway is fundamentally corrupt and undemocratic.

Indeed, you sometimes think there is some occult process at work that is driving Scotland out of the Union. It's as if the UK parties and the UK press just can't help themselves. Ruling out any deal with the SNP, complaining about "Scottish interference", is simply distancing Scotland even further from the UK. It is just playing into the SNP's hands.

There is an assumption that Nicola Sturgeon is itching to be part of the government of the UK, but that is far from the case. It doesn't particularly suit the SNP's game plan to be closely involved in Westminster governance. In a formal coalition they could become collectively responsible for some unpopular measures on public spending and defence that Labour might impose. The Liberal Democrats have been destroyed as a political force in Scotland by their alliance with the Conservatives.

The Scottish National Party is now looking increasingly to the 2016 Scottish parliamentary elections and doesn't want to do anything to damage their immense popularity in Scotland. When it comes down to it, Ms Sturgeon may have more to gain from trying to suggest Labour is in bed with the Tories than from getting into bed with Labour herself.