The issue of tax is already re-ordering Scottish politics

And yesterday brought a fresh attack when it was revealed the tax rise would mean someone on average earnings in Scotland (£27,710) would pay £162 more in tax than they would in England, if a Labour Scottish government got its hands on the newly devolved income tax powers in 2017/18.

Labour wriggled a bit over the first onslaught.

From the outset it had been clear the proposed rebate was only a stop gap measure until the new income tax powers came into force from April next year.

But in briefings issued when the plans were announced at the start of February, it gave the impression the tax system would be used to provide some extra protection for those on low incomes.

As it turned out, "using the tax system more directly" actually meant doing nothing at all.

In the Budget, George Osborne raised the personal allowance, the starting point for paying income tax, from £11,000 to £11,500, cutting bills for all basic rate payers. At the lower end of the income scale, that cancelled out Labour's 1p rise.

Kezia Dugdale can truthfully say the lowest earners will pay no more than they do at present under Labour's plans, but there is no longer any suggestion of additional help for them.

It was "a true betrayal of Labour's roots," said Nicola Sturgeon. Ouch.

Despite some painful headlines, Labour is not too distressed.

Tax is the issue it wants to talk about above all else in this election. And the fact these attacks are coming from both the SNP and the Tories rather tells Ms Dugdale's story for her.

It's easy to get bogged down in the detail of allowances, thresholds and rates but the big picture is clear enough. Comparing the present tax year with the next, when MSPs assume almost full control over income tax, Labour and the Lib Dems would increase income tax for almost everyone, with higher earners paying proportionately more.

Labour's commitment to restoring the 50p top rate means the biggest earners, those on more than £150,000, would pay quite a bit more.

The Conservatives would cut income tax significantly, with bigger earners in Scotland benefitting from the same increase in the higher rate threshold as their counterparts in England.

The SNP would cut income tax a little bit, raising the higher rate threshold only in line with inflation.

Labour accuses the SNP of failing to use Holyrood's new powers to raise the kind of revenue required to prevent further cuts to public spending.

The SNP says it is unreasonable to raise the basic 20p rate and reckless the raise the top 45p rate. The Conservatives say any changes that would make Scotland more highly taxed than the rest of the UK would be bad for business. That's the election in a nutshell.

And while the SNP attacks Labour over tax rises for lower and middle earners, Labour believes the SNP is vulnerable over its approach to higher earners.

Ms Sturgeon argues that raising the top rate to 50p would backfire, and actually cost money, if it prompted only a small number of the highest earners to move away from Scotland.

Yet the evidence that would happen is far from conclusive. Her second argument, that Holyrood has no powers to tackle tax avoidance, is even flimsier. Privately, Ms Sturgeon's advisers have concerns about HMRC but admit they have no evidence the taxman is failing to collect large chunks of income tax from well-paid Scots. This week HMRC issued a statement saying it was ready to "enforce compliance with tax laws".

If Labour can make a decent case for higher taxes and increased public spending, it stands a chance of reconnecting with former supporters who have switched to the SNP. How well Ms Dugdale does remains to be seen.

But it's interesting to see how the issue of tax is already re-ordering Scottish politics. For a long time now, the SNP has enjoyed considerable success presenting itself as a more authentically left wing party than Labour. In this campaign, though, Ms Sturgeon is making a virtue of her party's place in the centre ground.