What does John Swinney's response to Labour's tax rise say about his own policies?

His targets shuffled a little uncomfortably in their seats as he shared his delight at standing "shoulder to shoulder with the SNP in holding the line against the tax grabbers".

Of course, the MSPs loving this the most were not his Tory colleagues but those on the "tax grabber" Labour benches opposite him.

After months of being tarred with a Tory brush by the SNP, Labour MSPs were delighted to see a senior Conservative welcome the Nationalists to his fold.

Mr Fraser was talking about Scottish Labour's proposal to use Holyrood's existing tax powers to increase income tax by 1p across the 20p, 40p and 45p rates.

It dominated debate at Holyrood this week just as it will shape the coming election campaign.

For Labour, it is a progressive measure that will raise £475million to protect public services from deep cuts.

Mr Swinney has rejected it out of hand, saying it would "hit the taxpayers who are least able to pay".

All week the SNP has been attacking the move as unfair. What that might means for its own tax policies, I'll come back to.

The Resolution Foundation think tank analysed Labour's policy in an effort to bring a little clarity to the debate.

It started by stating the obvious: hiking income tax is progressive, meaning the better off would pay a higher share of their incomes in increased taxes than those earning less.

At the bottom end of the income scale, around 15 per cent of families would actually gain from the policy while half would be unaffected thanks to Labour's promise of a £100 rebate for everyone earning under £20,000.

The tax burden increases the higher people are on the income scale.

However, the think tank highlights some "hard cases," including the worker on £25,000 who would face a £140 tax rise.

It also notes that the rebate system tends to favour households in the top half of income scale.

This is because a couple earning nearly £20,000 - ie a household income of almost £40,000 - would qualify for £200 in rebates.

Overall, though, Torsten Bell, the think tank's director, concluded: "Taken as a whole this package is progressive. But there is an argument as to whether it is progressive enough."

The SNP's response to that last question has been a firm No.

Explaining his decision to stick with George Osborne's income tax rates last year, Mr Swinney told MSPs: "The income tax powers we currently have do not allow us to make income tax fairer, and I will not penalise the poorest taxpayers."

Effectively, he seems to be arguing that because he cannot shift the income tax burden a lot more firmly onto the shoulders of the better off, he doesn't want to shift it a little bit in their direction.

That's also illustrated by the SNP's argument that, as a percentage, lower earners' tax bills would rise by more than higher earners' under Labour's plan.

The tax paid by someone earning £20,000, for example would rise by five per cent, which is getting on for twice the percentage tax rise for someone on £200,000.

This does not alter the fact the tax rise is progressive. The person on £200,000 would see a bigger portion of their income disappear in tax after the increase than their colleague on £20,000.

But if the SNP wants to equalise percentage rises in tax bills, it could only do that by using powers proposed in the Scotland Bill to increase the higher rate more than the basic rate.

That could mean very significant income tax rises for higher earners in future years, especially if Mr Swinney takes on board another of the Resolution Foundation's conclusions: that to raise significant sums of money it would be necessary to increase the basic rate because of the relatively small numbers of higher and top rate payers.

Mr Swinney is under growing pressure to set out his tax plans and these are these are tough questions for him. They only apply, however, if a deal on the Scotland Bill is done and the new tax powers are approved. That is an increasingly big 'if'.