ED Miliband's decision to rule out any kind of formal pact with the SNP makes it easier to see how a minority Labour government would seek to operate after May 7.

As anxious Scottish Labour MPs were told before the election campaign, the strategy would be to defy the SNP to vote against Labour.

The Nationalists might be reluctant, knowing they would be accused of being wreckers or, worse, aiding the Conservatives.

Ms Sturgeon has already tacitly acknowledged this is how things would work out.

In an interview with The Herald last week, she accepted she would have to win the support of scores of Labour rebels if she wanted to force SNP policies through the Commons.

It means the SNP would be limited in what it could achieve but it would not be powerless, as it could withhold support from specific Labour policies in Commons votes.

In theory, that could lead to a series of trade-offs: SNP support on some issues in return for concessions on others.

But Labour would have cause to be wary.

Over the weekend, Conservative Home Secretary Theresa May and and Lib Dem leader Nick Clegg both claimed if Labour came second in the election but formed a government with SNP backing, the new administration would not be accepted as "legitimate" by a large section of the public.

A minority Labour government would need to win people's confidence by governing efficiently and effectively.

It could not risk becoming a soap opera of back-room deals, climb-downs and Commons defeats.

Mr Miliband would have build alliances where he could.

On the economy, for example, the SNP backs his tax plans and, despite much rhetoric to the contrary, shares a similar plan to cut spending and eliminate the deficit.

He might expect support from an SNP that struggled to find good reasons to oppose him.

On the other hand, measures that could not command a Commons majority would most likely never see the light of day.

It is a situation the SNP could easily exploit.

If an informal, uneasy and unpredictable Labour-SNP alliance emerges, it might provoke an "anti-Scottish" backlash south of the Border which could feed support for independence.

If the SNP finds itself on the sidelines more often than not at Westminster, outvoted on Trident and devo max, it will claim "Scotland's voice" was not being heard.

But will it cause "chaos and constitutional crisis" as Gordon Brown claimed at the weekend or "merry hell," as another former prime minister, Sir John Major, warned?

It might be in the SNP's long-term interests not to rock the boat too vigorously.

With the SNP leadership apparently cautious about holding a second independence referendum until it can be sure of victory, Ms Sturgeon's is promising to be a "constructive and responsible" ally for Labour.

Supporting a successful Labour government would go down well in Scotland. So well it could make Scottish Labour redundant in the eyes of many voters.

Her ultimate aim is to advance the cause of independence but that does not mean she has to wage all out war at Westminster from the word go.

We'll only really know where the balance of power lies when the votes are cast. But we do know this election is a lot more complex than sifting through a pile of manifestos and picking the one you like best.