Most politicians have recently sought to avoid dismissing the threat from UKIP, so it was striking to hear Danny Alexander declare on the Marr show this morning that "UKIP will come and UKIP will go".
It's unclear whether he had in mind the period before or after the next general election, but neither prediction is safe. First, while UKIP's support is likely to fall significantly in May 2015 (it is level-pegging with the Lib Dems on 11 per cent in today's YouGov poll), there's still a good chance it will poll above 5 per cent, a level of support that is high enough to determine the outcome of the election. At the last election, with a UKIP share of just 3 per cent, there were 20 constituencies in which the party's vote exceeded the Labour majority (one shouldn't make the error of assuming that all those who supported the party would have backed the Tories in its absence, but many would have done). UKIP's support has fallen from a peak of 16 per cent (with YouGov) in the weeks after the local elections but it will likely enjoy another surge when the European elections are held next May.
Second, while the return of the Tories to opposition, most likely under a more eurosceptic leader (it's plausible that the next leader of the party will support withdrawal), would hit support for UKIP, I expect it would remain a significant force. As the polls regularly show, anti-EU sentiment is not the main cause of UKIP's success and, in an age when all three of the main parties are untrusted, it could become a permanent receptacle for protest.
Under first-past-the-post, UKIP will always struggle to win seats, but I'm not as confident as Alexander that the Farageists will fade away after May 2015.