What is Cameron up to with Green Party debate offer?

Why is David Cameron throwing the Green Party a lifeline and potentially boosting their support at the next election by talking of bringing their leader into the TV debates?

He is trying to spook Labour because his own polling has uncovered what our own focus group suggested last month.

Lib Dem defectors to Labour have been what Prof John Curtice calls “Labour’s crutch.” They’ve allowed Labour to up its share of the vote without much of the traditional Con-to-Lab swing you might expect to see at the moment.

Squeezing down this psephological windfall is essential to Tory victory or largest party status.

I understand the Tories’ own polling and focus grouping has suggested to them that, though they’re strong for Ed M at the moment, some of these Lib Dem defectors might just slope off to the Greens come 2015.

They were, after all, more comfortable voting Lib Dem in 2010 when most of them probably thought the Lib Dems wouldn’t get into government.

After floating the idea of one debate including five leaders in the Sunday Times, David Cameron has now directly raised the idea in tv interviews on the campaign trail for the Local and European Parliament elections.

His aides emphasise that nothing has been formally agreed and all options are up for discussion.

Labour campaigners say their Lib Dem defectors are solid gains. Some psephologists agree and Lord Ashcroft’s polling a while back seemed to support that claim.

But the Tories obviously think some could shift and bring down Ed Miliband’s tally, making their own challenge a bit more surmountable.

Follow @GaryGibbonBlog on Twitter