So Labour is going to force a vote in the House of Commons on the mansion tax. It’s the key announcement trailed as part of Ed Miliband’s visit to Eastleigh today, and yet it’s the sort of thing that only really excites people who look forward to hearings of the Public Accounts Committee rather than the average voter who has a more normal perspective on life.

The idea behind these Opposition Day debates is that Labour flushes out any rebels or unhappy Lib Dems, or that its MPs can later tweet ‘Lib Dems voted against their own policy in the Commons tonight #evil #nevertrustalibdemagain’. Miliband says that very thing today:

‘And what about the Lib Dems? There could be a majority in the House of Commons when it votes on our proposal. But only if the Liberal Democrats vote with Labour. Now the Lib Dems say they are in favour of a mansion tax. Well, they once said they were in favour of abolishing tuition fees too.’

But the problem is firstly the one that I detailed above which is that these sorts of debates very rarely make stories and are often quite poorly attended by MPs on all sides. Secondly the motions that the Opposition tables for these debates are often interminably long and so over the top that it’s quite clear no Lib Dems would come wriggling out of the woodwork, even for their beloved mansion tax.

The Lib Dems are already rather amused by the idea. Stephen Williams, the party’s backbench Treasury spokesman in the Commons tells me:

‘Generally Labour MPs vote for Labour motions, and other people do not. They are doing just silly politics, this is the sort of thing that does not appeal to the public.

‘Tony Blair in 1994 and David Cameron in 2005 when he became Tory leader could credibly say I’m a new person, my party needs to change, I’m rebuilding it. But for Ed Miliband and Ed Balls, they were both in government when they abolished the rate, they simply have no credibility. We were talking about the mansion tax before the last election and Labour were trashing it back then.’

So this isn’t going to be what journalists like to call a ‘crunch vote’. The crucial day will of course be the Budget when the Coalition hopes to unveil yet another rise in the personal tax allowance, which as Ryan Bourne explained on Coffee House this week, is a far more effective method of helping low income households through the tax system than a 10p rate + mansion tax set up. The real crunch for the government is whether it’s able to make this case more effectively than Miliband by producing a Budget that doesn’t unravel so it can focus on this policy rather than on a series of U-turns.

Tags: Ed Miliband, Labour, Mansion tax, Tax, UK politics