Will that be the case with UKIP? They topped the European polls in May with 4.4m votes (27.5%) and record an average score in the range of 11-15.5% in local council by-elections. The polls tell the same story. When asked explicitly about general election voting intentions, 17, 18, 19 - even 21% are not unknown. How reasonable can we expect a vote collapse and an exodus that would disproportionately benefit the Tories?
I would be surprised if UKIP's vote dips beneath 10% next May (assuming Scotland decides to stay - a yes vote might boost the party further). There are a couple of reasons for this. First off, British politics entered properly new territory after the 2012 Corby by-election, the last 'conventional' parliamentary by-election this parliament. Since then, regardless of who holds the seat, UKIP have come second. This underlines their character as an anti-politics protest party with the capacity to do well everywhere as fed up voters succumb to temptation and give Westminster the finger. The drivers of anti-politics are well established, and need not be repeated here. That said economics, culture, and gender all play their part.
This anti-politics vote is pretty volatile. The LibDems used to benefit from its less virulent forms, as do the SNP, Plaid Cymru and Greens to greater and lesser extents. The BNP panicked official politics because it fished well from this pond for a short while. UKIP also have well before their current turn in the sun. Remember, Farage has been a professional politician since 1999. As is well known, this disproportionately affects the Tories but UKIP benefit from general discontent - they drink deep from the main parties' electoral run-off. Unfortunately, most political comment operates as if this "run-off" is all there is. UKIP attract more Tory supporters because they too are a right-wing party, albeit one unafraid of putting the boot into immigrants and Europe.
For most of UKIP's history, this has been the case. But since the Coalition was formed, Farage and friends have benefited from an organic crisis in the Conservative Party. They have demographic trends against them, whole swathes of the country are electoral no-gos, the Tory grass roots have yellowed and withered, and they're so beholden to the city that as a party of business, they are outright dysfunctional for British capital-in-general. Dave's half-arsed "modernisation" - embracing equal marriage and equality for LGBT, women, and minority ethnicities, and faux toughness toward Europe - has exacerbated long-term decline and delivered activists by the blue-rinsed brigade load to UKIP. He hasn't really moved away from the Thatcherite Toryism his misanthropic membership hold dear, but it appears otherwise.
Because UKIP's growth is underpinned by the crisis of British conservatism, a lot of these people will not be going back. They are fundamentally and irreconcilably alienated from the Conservatives' direction of travel. That's why anyone betting on a collapse in UKIP support, like a number of "professional" commentators I could mention, is very foolish indeed.
Don't take it from me, though. Look at the evidence. Cast your mind back to the early 1980s. Labour was bingeing on internal strife as factions duked it out in a no holds barred battle for the party's direction. In March 1981, the so-called Gang of Four - David Owen, Shirley Williams, Roy Jenkins and Bill Rodgers led a right-wing split from Labour. The new Social Democratic Party could eventually count on a parliamentary phalanx of 28 former Labour MPs (and one Tory). It would stand as the Alliance (with the Liberals) in 1983 and 1987 before merging and creating the modern Liberal Democrats in 1988. The SDP attracted all kinds to its banner but, as you might expect, it was most appealing to soft and moderate Labour supporters - just as UKIP does to hardened Tories. Furthermore, the SDP's support did not evaporate in the 1983 general election. Compared with 1979 (where the Liberals got just shy of 14%) the SDP arguably brought about 10% to the Alliance's vote tally. It was less the Falklands and "the longest suicide note in history" that did for Labour. It was the product of their own crisis: the SDP. Subsequently in 1987 the SDP vote held up, the Alliance slipping by just 2.8%. The SDP performed well because it was an outcome of the organic crisis in Labourism.
Electoral precedent is no iron law, but the British electorate have form for voting in substantial numbers for political alternatives under certain circumstances. After the crisis in Toryism exploded into the open, looking back at UKIP's general election scores for 2010 and 2005 is misleading. Now the present day party is as much a product of crisis as the SDP was, what happened back then is a better indicator of UKIP's performance next May. Add that to hundreds of thousands of voters in the habit of voting for them in second order election after second order election, those polls might not inflate UKIP support by a great deal at all. UKIP's vote is not going to collapse. There will be no significant drift back to the Tories.