With exemplary timing, Labour has chosen the opening day of the Lib Dem spring conference to publish the text of its Commons motion in favour of a mansion tax. Nick Clegg can now expect to be challenged over the weekend to say whether his party will vote in favour of it when the debate is held on Tuesday.
The motion reads:
That this House believes that a mansion tax on properties worth over £2million, to fund a tax cut for millions of people on middle and low incomes, should be part of a fair tax system and calls on the Government to bring forward proposals at the earliest opportunity.
The decision to exclude any reference to the reintroduction of the 10p tax rate (which Labour's mansion tax would fund), in favour of a vaguer commitment to "a tax cut for millions of people on middle and low incomes" (which could encompass a rise in the personal allowance), means it will be harder for the Lib Dems not to support it. Vince Cable previously suggested that his party would back the motion provided that Labour did not engage in "party political point scoring" and "drag in other issues like the 10p rate".
It depends entirely how they phrase it. If it is purely a statement of support for the principle of a mansion tax I’m sure my colleagues would want to support it.
But very often in these opposition days they can’t resist the temptation to make party political point scoring and drag in other issues like the 10p rate and if that happens I am sure we will not. It is up to them to be statesmanlike and sensible.
Clegg similarly refused to rule out voting with Labour ("Neither Vince nor I know what will be put before us so we can't of course determine in advance how we would vote"), prompting David Cameron to say that he would be "rather disappointed" if his deputy did so. He told ITV News: "I haven’t asked him the question. But as it’s not in the Coalition Agreement to have a mansion tax, I would be rather disappointed if he did."
For Labour, this is a win-win situation. If the Lib Dems back the motion, Miliband will attack the coalition as divided, while painting the Tories  as the party of the rich. If the Lib Dems oppose it or abstain, he will accuse Clegg's party of lacking the gumption to even vote for its own policy. As shadow financial secretary to the Treasury Chris Leslie said today:
If Nick Clegg and Vince Cable really believe in a fairer tax system they should back our motion in support of a mansion tax on pro perties over £2 million to pay for tax cuts for millions on middle and low incomes.
After going along with a Tory tax cut for millionaires, a failing economic plan, a VAT rise and a trebling of tuition fees this is a chance for the Liberal Democrats to finally vote for something that was in their manifesto.
With the Lib Dems already far from short of political anxieties, Labour has just created another dilemma for Clegg's party.