Since Theresa May called the snap election, there has been much discussion about encouraging those who are unhappy with her vision of Brexit to vote tactically to defeat those Tory (and occasionally Labour) MPs who back the prime minister. Voters, it has been argued, should form their own “progressive alliance”, voting for whichever anti-hard Brexit candidate seems best placed locally to defeat their pro-Brexit MP.

But while progressives have talked, those on the other side of the fence, who back Brexit, have walked. Without prompting, many a Ukip supporter seems to have decided that the best way of realising their vision of Brexit is to back May’s Conservatives. As a result, stopping the prime minister in her tracks has got that bit harder.

In four polls it conducted in March and the first half of April, ICM on average put the Conservatives 18 points ahead of Labour. But in five polls conducted in the last fortnight that lead has increased to 20 points. Other polling companies have found a similar trend. However, the increase in the Tory lead has not been occasioned by any drop in Labour support. In fact, the party’s support has edged up slightly, from 26% to 27%.

At the same time Ukip support has fallen by three points. This is no coincidence.

Before the election announcement nearly two-thirds (64%) of those who voted Ukip in 2015 told ICM that they would vote for the party again. Around a quarter (26%) said they would vote Conservative. Very few of those who backed the party in 2015 said they would vote Labour (6%) or the Liberal Democrats (2%). Now, however, only half of those who voted Ukip two years ago say they would do so again. The proportions who say they would vote Labour or Liberal Democrat have barely changed. But as many as two-fifths (41%) say they will vote Conservative.

Can we trust the polls? Most polls wrongly predicted the outcome of the last general election in 2015. This may have been because people changed their mind at the last minute or they were what has been referred to as “shy Tories” – those who felt uncomfortable revealing to pollsters that they intended to vote for the Conservatives. Before the 2015 election, both Mori and ICM polls came up with leads of one point for the Conservatives and one point for Labour. An Ashcroft poll on the day of the election showed a three-point lead for the Conservatives and a YouGov poll on the day suggested a dead heat. The last Populus vote gave Labour a one-point lead. In the end, the Conservatives had a seven-point lead. During the EU referendum campaign, polls did indicate a possible leave vote in the first weeks of June, but edged back to remain in the final days before the vote. Just two of the six polls released the day before the referendum, those carried out by TNS and Opinium, gave leave the edge. This is why Jeremy Corbyn has insisted Labour has a chance of winning despite poor opinion poll ratings.

This switch happened in almost an instant. When ICM polled over the Easter weekend, only 21% of 2015 Ukip voters said they would now vote Conservative. When the company polled in the hours immediately after Theresa May unveiled her election surprise, that figure leapt straight away to 39%.

ICM’s figures show that the Conservatives had already acquired the support of just over half (53%) of leave voters, even before the ranks of Conservative voters were swelled by this latest influx of Ukippers. But the figure now stands at three-fifths. Some other companies, such as YouGov, put the figure even higher. May has become increasingly successful at gathering last year’s Brexit vote under her own roof.

The resulting three-point swing from Ukip to Conservative costs Ukip itself little more than its pride. With Douglas Carswell gone, the party has little chance of winning a seat. But three more points added to the Conservative tally could easily boost May’s majority by some 24 seats, thereby increasing her chances of winning the large majority she evidently craves.

Moreover, the Ukip-to-Conservative swing may not yet be at an end. There have been reports that the party may not stand in some seats where a strongly pro-Brexit MP has a relatively marginal seat. In truth, given their party’s position in the polls it looks as though most Tory MPs will not need such help. Ukip might be better advised not standing against some strongly pro-EU Labour and Liberal Democrat MPs who are facing a pro-Brexit Conservative challenger. But wherever Ukip eventually decides to stand down, it is the Conservatives who seem most likely to profit.

While Theresa May gathers together the leave vote, the remain vote is scattered. According to ICM, 38% would vote Conservative (a figure that has not changed since the election announcement), 35% Labour, and 16% for the Liberal Democrats. All the talk about a progressive alliance is in truth an implicit acknowledgment of the fragmented nature of the remain vote. Moreover, many a Conservative voter who voted remain seems to have done so without a great deal of enthusiasm. Only one in three now say that there should be a second referendum or say that the UK should not contemplate leaving at all. Most thus look likely to remain loyal to the Tories.

Yet the increasingly pro-Brexit character of the Conservative vote does raise an interesting question. Big business and the City have traditionally been the Conservatives’ allies. Yet there must be some qualms among their ranks about the prospect of an election result that could give Theresa May a strong mandate to end freedom of movement and take Britain out of the single market. Could this see some big campaign money leave the party? Or do they think that “strong and stable” leadership means being better able to negotiate a softer Brexit, irrespective of what May’s voters seem to want?