Nigel Farage has announced that he will field Brexit party candidates across Great Britain. A blow to Boris Johnson’s re-election hopes? That’s the consensus across much of Westminster. Are they right?

Well, voters are volatile and it’s a six week campaign. Anything could happen. But we’ve had two elections in the last four years: the first, in 2015, saw Nigel Farage’s Ukip get close to four million votes, which were predominantly drawn from David Cameron’s 2010 vote, but an election-shifting minority of them came from Labour’s 2010 coalition, which made the difference in a number of seats across England and Wales, the most high-profile of which was Ed Balls’ Morley and Outwood seat.

As one Labour organiser told me at the time, the Ukip vote can split into four distinct groups: half of it is drawn from affluent Conservative voters and is mostly concentrated in safe Tory seats where they can afford to lose voters, a quarter of it comes from working-class Tory voters who are spread throughout the country, but the remaining quarter comes from Labour and is dangerously concentrated in areas where their voters really matter thanks to first past the post. The Conservatives' impression of where the Brexit party vote comes from is essentially identical, as we'd expect.

In 2017, Nigel Farage went into a brief retirement and Ukip were wiped out almost everywhere: the Conservatives lost their majority, because while for the majority of Brexit party voters their electoral preferences are “1. Nigel Farage 2. The Conservatives”, for a critical minority, their preferences are 1. Nigel Farage, 2. Labour 3. Gargling bleach 4. The Conservatives”.

That second group of voters, denied the opportunity to vote for their first preference, backed Labour and helped contribute to the 2017 hung parliament. Don’t forget that, as far as voters who have changed their preferences are concerned, “Leave 2016, Labour 2017, Brexit party 2019” outnumbers “Leave 2016, Labour 2017, Conservative 2019”.

If I were Boris Johnson, I would reason that I am or near the limits of my ability to win back voters from the Brexit party, but Nigel Farage’s outfit might well be a useful holding pen for Leave voters who would otherwise vote Labour.

2019 is not 2015 or 2017, of course. One important difference is that it didn’t really matter in 2015 that Cameron lost a bunch of affluent authoritarian voters to Ukip because he was able to pick up a group of affluent liberal voters from the Liberal Democrats. These voters had the added benefit of being much better distributed geographically speaking, unlocking a chunk of seats in the South-West of England.

It may be that while Labour still has reason to fear the loss of that quarter of the vote in traditional marginals, the Conservatives have more to lose now that the tide of pro-Cameron, pro-Remain voters is reversing somewhat.

The reality is that we don’t know for sure either way. As dull as an aphorism as it is, the one thing we can say for certain this side of a general election is that Nigel Farage’s decision to stand candidates across the United Kingdom will change things, but we don’t know exactly how. We have an old electoral system designed for two-party politics that is poorly equipped to handle anything more complex, and voters are more volatile than ever before.