UKIP will soon have to decide if they want to be a party or a pressure group

With Cameron's EU referendum promise (by 2018) yesterday on a renegotiated EU membership for the UK the "tectonic plates" of British politics have now shifted again.

UKIP deserve credit for having contributed, perhaps decisively to Cameron's decision to opt for this plebiscite. It's even more impressive when you consider that they do not even have any MPs.

However the announcement could provoke an existential crisis within the new upstart right-wing party. Their main (although by no means only) policy is for an in/out referendum on Europe. It's why the party was formed and is really the only policy that they could honestly say their entire party will coalesce around. Cameron has shot that particular fox.

Come 2015, UKIP will have an important decision to make. Should they fight tooth and nail for every seat in the country as a truly national political party would? The risk with this is that it splits the vote on the right and could directly lead to the Tories losing the election. With Miliband already on the record against an EU referendum the effect of that could be to prevent the very referendum they want. Our bizarre electoral system could mean that a substantial vote for UKIP means that ironically we end up further away than ever from a vote on EU membership.

So maybe they ease off on the gas a little, especially in Conservative seats. But that is not what a true political party would do. Surely UKIP members, certainly in those blue seats will start to wonder why they are bothering if they are (either tactily or even overtly) asked to rein it in for 2015. Expect a fair bit of leakage back to the Tories of both voters and members if that was to happen threatening UKIP's viability in the longer term.

The other possibility is that there is some sort of pact with the Conservatives. I think this is unlikely but if UKIP call their bluff and continue to push hard Cameron may feel he has no choice but to entertain the idea. This would be a disaster for the smaller party. They would effectively be subsumed by the much larger party and whilst they may get a few MPs there will be huge swathes of UKIP supporters all over the country who are denied a chance to vote for a UKIP candidate. Again, not the actions of a party that wants to maintain widespread and popular support.

I don't know which way things will go but I know that however they do play out UKIP now face a threat to their very existence.

I'm sure this was at least a little bit in Cameron's mind when he made his speech yesterday.