LONDON — After soaring high in British politics, the United Kingdom Independence Party, which played a key role in forcing a referendum on EU membership and then a vote for Brexit, is now braced for a crash landing.

The harsh reality for UKIP is that since the June 2016 referendum its revolt on the right has stalled and many traditional Tory voters who shifted their support to UKIP are now heading back in the other direction. That's in large part down to Prime Minister Theresa May's repeated insistence that "Brexit means Brexit," her prioritizing of immigration reform, criticism of liberal elites and push for more grammar schools — all of which will be music to UKIP voters' ears.

The effects of this shift are clear. Since May called the general election, UKIP’s average share of the vote in the polls has slumped to 7.2 percent, well down on the almost 13 percent that the party polled at the 2015 general election and the 17-19 percent it was polling ahead of the referendum last June.

There are also rumors that UKIP, which launched its election campaign on Friday, will field far fewer candidates than it did in 2015 (when it had more than 600) and will not take on Brexiteers — a move that signals a return to a similar, unofficial, pact with Conservative Euroskeptics in 2010.

The rise of UKIP

In 2015, UKIP delivered one of the most impressive performances by a challenger party in British history, winning 3.8 million votes or 12.7 percent. Yet the party failed to attract enough votes in the right places, a key requirement in the U.K.'s first-past-the-post electoral system.

Almost four million votes were rewarded with only one seat in Westminster — the economically struggling coastal constituency of Clacton in Essex. The man who won the seat, Douglas Carswell, joined UKIP from the Tories but quit the party last month to become an independent MP and isn't standing in the June 8 election.

As David Butler, the godfather of British election studies, observed at the time, it was "the harshest treatment that our capricious electoral system has ever inflicted on a nationwide party."

UKIP typically polled at around 14 percent of the vote in 2015 but across the party's top target seats this jumped to 30 percent. The party polled at least 10 percent of the vote in 450 seats and more than 30 percent in eight seats, including Clacton and other working-class areas.

The party finished second in 120 seats. (It finished second in just one constituency in 2010.) Of these second place finishes, 76 were in seats won by the Conservatives and 44 in seats that were won by Labour. In 35 northern England seats where UKIP are currently second, all but one are held by Labour.

What will happen to these votes in June? One way of answering this question is to use the British Election Study, which collected data in December 2016 — that's just after Paul Nuttall became party leader and before UKIP’s rapid decline in the polls after the 2017 election was called.

If anything, what follows are conservative estimates.

First, there is clear evidence that lots of UKIP voters are switching over to the Conservatives. Of those who voted UKIP in 2015, 58 percent said they will vote for the party again at the next election while 42 percent said they would switch to another party or did not know how they would vote.

Of those who plan to abandon UKIP for another party, around 73 percent are switching to the Conservatives and just 14 percent to Labour.

But how likely is it that the 58 percent of the UKIP electorate will remain loyal? The short answer is "not likely at all."

One way of exploring how committed UKIP supporters are is to ask them how likely they are to switch to another party. Doing so reveals that, on top of those who have already defected, 25 percent have a high probability of switching to the Conservatives (compared to just 3-4 percent who have a similar probability of switching to Labour or the Liberal Democrats). Similarly, if we then look at voters who currently do not know how they will vote in June, 1 in 4 have a high probability of voting Conservative. This suggests that May is about to become a major beneficiary of UKIP's collapse.

How will all of this play out in the key Conservative-Labour battlegrounds in June?

Given that up to one-third of UKIP voters are planning to defect to the Conservatives, it could have big implications for the Labour Party. If the one-third figure is correct, at least 16 Labour-held seats will be directly at risk.

Labour's problems don't end there. Let’s loosen our conservative estimates a little and assume that Prime Minister May wins over a larger portion of UKIP's electorate. This seems plausible given the further decline of support for UKIP in the polls and recent polling in Wales, which points to major gains for the Conservatives that partly flow from a collapse in public support for UKIP.

If we assume that 50 percent of UKIP's 2015 voters switch to the Conservatives, then at least another 15 Labour MPs — that's 31 in total — are at risk. This includes Labour MPs across Wales as well as in industrial, blue-collar seats in England.

Lib Dem revival?

The good news for May keeps on coming. UKIP defections to the Conservatives are also most likely in seats where she faces a strong challenge from the Liberal Democrats. Nearly one-quarter of UKIP’s 2015 voters in these seats now intend to vote Conservative, with those remaining loyal to UKIP dipping to roughly 50 percent.

If we then exclude voters who do not know how they will vote, a striking 86 percent of UKIP defectors have gone to the Conservatives. There is also little evidence that Conservative Remainers are switching over to the pro-EU Liberal Democrats, which you would expect to see if this really were a realignment election.

So the Liberal Democrats face a double whammy: A strong Conservative Leave vote that is being reinforced by ex-UKIP voters who trust Theresa May to deliver Brexit, and soft Remainers, who appear to have accepted the referendum outcome, being unwilling to jump over from the Conservatives.

For these reasons, the prospect of the Liberal Democrats — the party slumped to just 8 seats — overturning their 2015 losses to the Conservatives look bleak while the prospect of May benefitting from UKIP’s collapse looks far more certain.

The fall of UKIP, and the numbers presented above, are one reason why current forecasts put May’s estimated majority at 130 seats — a position of strength in parliament that the Conservative Party has not experienced since Margaret Thatcher’s victory in 1983.

Britain is preparing for a realignment on the right.

Matthew Goodwin is professor of political science at the University of Kent and author of "Brexit: Why Britain Voted to Leave the EU." He tweets @GoodwinMJ. David Cutts is professor of political science at the University of Birmingham.