The decision by the Lib Dem leadership to allow David Davis to resign in the comfortable knowledge that no Liberal Democrat would oppose him was a strange one. Without Lib Dem acquiescence, the by-election almost certainly would not have taken place. As it is, it seems unlikely to me to do much to fracture the Tories’ current revival, and it may allow the party’s opponents to gain valuable ground, and not just in Haltemprice and Howden.

Who, then, might do well enough in the by-election to gain ground while the Liberal Democrat by-election machine twiddles its thumbs? Could someone from the ranks of the Independent candidates gain votes on the ‘42 days’ issue? Ealing vicarage rape victim Jill Saward has attracted some publicity for her stand. Or the horrible National Front (neither the BNP nor UKIP are standing) might do well by attracting immigrant-haters who disagree with Davis’s stand. Or perhaps no-one will seriously trouble him, since The Sun failed to stump up the money for Kelvin Mackenzie to take Davis on.

However, my own view is that the Green Party is best placed to use the Haltemprice and Howden by-election to gain ground. They have far better civil liberties credentials than David Davis. He is pro-death penalty, pro-28 day detention, against the Human Rights Act, and supported the Government ban on demonstrations within a mile of Parliament. Giving the Greens a free run against him in this by-election has allowed them the opportunity to test their potential in a race where they can present themselves as the only left-leaning opposition to the Tories.

And the Greens are becoming better organised at first-past-the-post politics. They managed 22% and a close third behind Labour and the Tories in Brighton Pavillion at the last election. And at local government level in Norwich, where the Lib Dems ran City Hall as recently as 2006, they are now the official opposition to Labour. All of the Green gains have been deliberately targeted at Lib Dem expense, and all in what should have been a strong Liberal Democrat Parliamentary prospect at the next election. The Greens are no doubt campaigning hard in Haltemprice and Howden, reinforced by clever campaigners from Norwich, Brighton and elsewhere.

And while much comment about the Henley by-election has centred on Labour’s dreadful fifth place, hardly anyone seemed to notice that the Green Party came third. Their vote share, with a strong Liberal Democrat campaign, was less than 4%, but it will be interesting to see how much the Greens can improve this share with no Liberal Democrat effort at all.

Their immediate task has been a difficult one, given the short notice of this by-election – less than a month from Davis’s resignation to polling day. They may not have had the time to make a very significant impact in a seat they have never contested before. So a Green win on Thursday is very unlikely. Even on a very low turnout, with Green student campaigners just starting their long vacation. Even roundly attacking Davis’s record on the very liberties he claims to be upholding. And even with the Tories divided over Davis’s stance, but complacently assuming he will win easily anyway.

In such circumstances however, a good second place does not seem outrageously improbable, and may attract national media attention. It may act as a catalyst for further progress. Liberal Democrats leaders may come to regret giving them the opportunity.

* Terry Gilbert is a former Liberal Democrat Parliamentary candidate, and has been a Lib Dem member since 1983.