David Cameron has only himself to blame. Having won the leadership of his party by appearing to be at least as Eurosceptic as his rivals, he is now threatened because he has not fulfilled the implied promise of retracting the UK from the EU. Never mind that he recognises the value to Britain of staying within the Union (and probably had some inkling of this even before he came into government) and that he knows that the Conservatives need to appeal to a broad swathe of the UK electorate in order to succeed at the next election, the little Englanders in his party are out to get him. I have only the Indy's report to go on, but it appears that Michael Fabricant is in their number. He believes that the Conservative party is now structurally Eurosceptic and that an alliance with UKIP is necessary. There is clearly pressure on Cameron to take a harder line on our links with the continent. There is implied pressure to deselect EU-realist Conservative MPs.
All this is music to the ears of rival party leaders. The Conservative party could become more extreme - or more split. The UK electorate does not like extreme or split parties. The real threat from UKIP is not so much in terms of votes. UKIP has probably garnered as many genuinely anti-Europe votes as it is likely to reap in 2015, in spite of support in the London press for its stance. UKIP's recent gains are as likely to be protest or anti-politician votes, as Professor Curtice points out in a side-bar to the Indy article quoted above. The greater danger for the Conservatives in my opinion is that they will lose their activists. Just as Labour party workers tend to be more socialist than their leadership and Liberal Democrats more liberal and democratic than theirs, so the average Tory constituency worker is more class-conscious and jingoistic. UKIP's domestic manifesto - leader Nigel Farage is now stressing that they are no longer a one-issue party - will appeal to those Tories also.
Labour should benefit in the relatively few constituencies which are Labour/Conservative marginals. However, unless there is some honesty in Labour campaigning in 2015, any Labour gains will we wiped out by Conservative gains over Liberal Democrats. If Labour campaigns hard for its candidates in seats where it has consistently come third, and against coalition, as it did in 2010, then the anti-Tory vote will be split where it counts most. If Labour wants cooperation with LibDems after the next general election, it would help if their leadership recognise publicly the more progressive measures which our parliamentary party have managed to achieve through the coalition, and the restraint which we have managed to impose on the most reactionary Tories.
Later: the Liberator collective has an interesting sidelight on the appeal of UKIP.