Tonight will see the outcome of the most keenly watched and hotly debated by-election (i.e., special election for a vacant constituency) in Britain since the rise of the United Kingdom Independence Party. It was caused by the defection of Mark Reckless, the Tory MP for Rochester and Strood, to UKIP and by his further decision to resign from Parliament and re-submit himself to the electorate in his new party colors. That second decision followed the example of Douglas Carswell, an earlier Tory defector to UKIP who went on to win a landslide victory at his Clacton constituency, and it has a nice 19th-century ring of honor and principle about it. It’s one of those features of UKIP that, at least in the eyes of the voters, set it apart from a self-interested professional political establishment at Westminster.

David Cameron’s Tories were philosophical about the loss of Clacton. Carswell was a very popular local MP, and they told themselves that Clacton was a backward, failing, depressed sort of place, a seaside town that the future had left behind, in short natural UKIP territory. Matthew Parris, the witty columnist for the London Times, who once worked for Mrs. Thatcher but is now firmly in Cameron’s corner, wrote an entertaining, clever, brave, if somewhat snobbish piece on the theme that the Tories should turn their backs on Clacton, or at least on its opinions, in favor of more forward-looking places such as Cambridge. Clacton is, he wrote, ”Britain on crutches . . . tracksuit-and-trainers Britain, tattoo-parlour Britain, all-our-yesterdays Britain. So of course UKIP will do well in the by-election.”

And so UKIP did. Carswell won more votes for UKIP than all the other parties put together.

Rochester and Strood was supposed to be different, however, and the Tories were initially confident that they would do well there, even holding the seat against the UKIP challenge from their former colleague. The reason was: R&S wasn’t Clacton. In almost every indicator of social and economic well-being, R&S is either just on or slightly above the national average. Its house prices, its wage and salary levels, its employment figures, its standards of educational and professional attainment, even its opinions on contentious issues such as immigration and the European Union—these are all uncannily close to those of Middle England as a whole. If UKIP can win here, it can win anywhere.

As the campaign has progressed, however, the polls have been forecasting that not only can UKIP win in R&S but that it is on course to do so quite handsomely—maybe by a margin of as much as 10–12 percent. Accordingly the Tories have reduced their own expectations dramatically and now let it be known that they would be happy to prevent an actual UKIP landslide. A mere victory for UKIP, they argue, would signify nothing more than a transient solid protest vote that would be reversed in next May’s general election. At the same time their tactics in the campaign have undermined this seeming London assurance: They have been a series of attempts to out-UKIP UKIP.

Thus the local Tory association, imitating UKIP’s democratic populism, announced that it was holding an open mail-in primary to select their candidate from among two local women councilors. That represented a very moderate infusion of democracy and inspired very little excitement. Only 5,700 people returned the ballot papers that had been mailed to every voter in the constituency. It was not hard to see why: There was going to be a regular election with many more candidates that could elect an actual MP in less than a month. Voting in the primary didn’t seem worth the price of a postage stamp.

Then the Tories decided to copy UKIP’s appeal as a movement of the people struggling against the Westminster LibLabCon establishment. They issued a leaflet contrasting the simple homespun candidate they had chosen, Kelly Tolhurst, against the sneering upper-class establishmentarian on the other side, Mark Reckless. In its way the leaflet was a brilliant parody of UKIP’s usual propaganda: Kent girl versus carpetbagger, campaigner on local issues versus Westminster sophisticate, girl from local academy versus Oxbridge graduate in politics, small businesswoman versus banker! Gad, implied the local Tories, how does the fellow dare show his face here. Horsewhipping is too good for him. Unfortunately for this pitch, as commentators noted, the local Tory constituency had selected Reckless to be their candidate for three contests running. It was a little late for them to discover that he was a Victorian stage villain.

In the final week, the Tory attempts to turn back the UKIP wave have looked increasingly desperate. David Cameron personally appealed to Labour voters to stop UKIP in order to preserve “stability.” A Tory leaflet alleged that if UKIP won, the price of their houses would go down. That’s the nightmare of every suburban voter, but it suffers in this case from being false even by the standards of election leaflets. But there’s tough competition for that award. Tory MPs visiting R&S to campaign complained that the leaflets they were given to hand out on the doorstep suggested that “uncontrolled immigration” led to crime. That was objectionable, they thought, on two grounds: First, it didn’t necessarily lead to crime, and second since the Tories have been in government for the last five years, who was to blame for this uncontrolled immigration? Finally, in the last few days the amiable Ms. Tolhurst issued a leaflet blaming Cameron himself for not being tough enough on immigration control. No one thinks that Ms. Tolhurst wrote this letter. A candidate in an important by-election is no more than “a bloody legal necessity,” whose campaign is run entirely by the professionals at the Conservative Central Office. So the Tory party is now attacking its own leader in order to give voters the impression that it will in future cut immigration to lower levels than is in fact the case. All that remains is for David Cameron to reveal that he himself voted UKIP last time and is very dissatisfied with its performance in government. Don’t rule it out.

On the other side, UKIP has run its first really professional campaign. Polls suggest that voters have received slightly more visits from its canvassers than from those of any other party. Reckless himself has belied his name and run a quiet campaign with only one serious misstep. And the consensus of both politicians and political journalists—for what it is worth—is moving slowly towards the view that a UKIP victory on Thursday night would presage a new party structure in next May’s election.

Unless, of course, Rochester and Strood is not Clacton and the circus antics of the Tory campaign unexpectedly saves the day. Tune in tonight.