Continuing their final-straight sprint to join the main-party pack ahead of the general election, the Greens have enjoyed more good press in the last few days.

Not least the news that they have reportedly overtaken UKIP’s membership, with demand reportedly sufficient to crash the party’s computer system.

On top of that we have the latest Lord Ashcroft poll showing a three-point advance in the party’s support, placing it ahead of the beleaguered Liberal Democrats and just behind UKIP – a rise credited in part to the Prime Minister’s attempts to include them in the TV debates.

But if he did get them in, how much would they damage Labour?

Over the past couple of years there has been plenty of analysis of the rise of UKIP and its potential impact on the major parties, most famously in Matthew Goodwin and Rob Ford’s Revolt on the Right and its Fabian response, Revolt on the Left.

The direct damage to Tory electoral prospects of UKIP’s insurgency, and increasingly its long-term threat to Labour heartlands, are thus increasingly well understood by commentators and political strategists.

The Greens have yet to receive such scrutiny, and there is likely not time between now and the general election for any comprehensive research.

In trying to shoehorn them into the TV debates, Cameron hopes that they will live up to the moniker “the UKIP of the left”, and analysis of their support in polling returns does suggest that they have a disproportionate impact on Labour.

This could be because, in addition to those who genuinely support Green policies and those looking for a protest vote, they attract support from left-wing voters who feel that in the post-Blair era the three main parties are much the same – the same charge levelled, from the opposite direction, by UKIP.

With Labour pursuing Ed Miliband’s ambition-starved 35 per cent strategy, anything which tempts those vital Lib Dem defectors out of the red tent could prove very painful indeed.

But First Past the Post means a party needs to do more than leech national share to seriously threaten one of the big three. UKIP’s potency lies in the fact that it takes votes off the Tories in close races where their impact is really felt – and it’s hard to imagine there are nearly so many Green voters in places such as Bolton West.

Moreover, what data we have suggests that Green voters are younger than average, and as Peter Hoskin points out today that usually bodes ill for turnout (although this may be balanced by their being drawn predominantly from high turnout social grades).

Thus despite the TV debate row increasing the party’s prominence, it is very hard to assess how much direct damage the Greens will actually do to Labour in May.

But even if its vote is not distributed as potently as UKIP’s, it could still play a useful role in dragging Miliband off course.

As the New Statesman reports, the UKIP comparison has Labour transfixed on the threat to its left. Along with the SNP, it could force Labour into the impossible position of trying to sate the hard left whilst trying to build credibility in the centre – and leave a space for an upbeat, ambitious Conservative Party to exploit.