The Tories and the Lib Dems have never sought to hide their differences over a mansion tax. But Nick Clegg went further than ever last night when he accused his coalition partners of "turning a blind eye to the super wealthy". He told ITV News :
The Conservatives need to speak for themselves. I for the life of me don't understand why the Conservatives think it's ok that an oligarch can buy a palace in Regent's Park for tens of millions of pounds and pay the same council tax as a three-bedroom family house in Lewisham.
That is just unfair. We can't keep turning a blind eye to the super wealthy basically being taxed the same way on their properties as hard working families across the country.
Clegg's words prompt the question of whether the Lib Dems will line up with Labour if and when Ed Miliband succeeds in forcing a Commons vote  on a mansion tax. Vince Cable has suggested that his party will vote in favour  of the opposition motion provided that it is not tied to the reintroduction of a 10p tax rate (which the Lib Dems oppose) and Clegg similarly indicated that it would depend on the wording.
Neither Vince nor I know what will be put before us so we can't of course determine in advance how we would vote.
But of course the Liberal Democrats for a long time have been the leading advocate of greater fairness in tax.
I've been told by a Labour source that the motion will not include a commitment to introduce a 10p tax band in order to maximise the chances of support from the Lib Dems. The party sees the vote as a chance to show how the Tories are on the wrong side of the new tax divide in British politics. Miliband believes that the Conservatives' decision to write privately to their donors soliciting funds to combat a "homes tax" leaves them particularly vulnerable to the charge that they are the party of the rich.
The irony is that before the last Budget, George Osborne, the man now leading the charge against the tax, considered introducting  two or three new higher council tax bands on houses worth more than £1m, a measure that the Lib Dems could have presented as a mansion tax. But this option was ruled out after David Cameron's shire Tory instincts asserted themselves and the PM personally vetoed the proposal. With the Tories now having ruled out anything resembling a mansion tax, the Lib Dems see no reason to go easy on their coalition partners.