LONDON — Boris Johnson, on a mission to “get Brexit done,” has given the U.K. its first December election since 1923.

On the face of it, the outcome of the vote looks set to relieve the U.K. from the uncertainty that’s gripped the country since the 2016 referendum to leave the EU, some 41 months ago.

Johnson’s Conservative Party, polling at 37 percent, has a comfortable 13 percentage point lead over the Labour Party. On those kinds of numbers, the Tories might expect a comfortable majority — enough to pass the Brexit deal the government negotiated with the EU earlier this month.

The trouble for the Tories is that support in the polls today doesn’t necessarily translate to votes in December. Whether the party can maintain its lead in the next five weeks — and how that lead will translate into seats — is far from certain.

There are several reasons why no one should claim to be able to confidently predict the result of the coming election.

While the 2017 election saw the Conservative and Labour parties winning 84 percent of the vote, the latest polls suggest these two parties would now win just over 60 percent of the vote.

First, voters are now more likely to switch parties than at any previous point in post-war British history. The British Election Study team has shown that in the last two general elections — in 2015 and 2017 — between one-third and two-fifths of voters switched parties between elections.

Often, it’s hard to predict who these switching voters will be. In 2010, the Labour Party counted Scotland as one of its heartlands, for example. In 2015, a very large proportion of its voters switched to the SNP. It’s not guaranteed that the next election will see as many voters switch again — but over the past four years many voting habits have been broken, and new habits take time to establish.

Current polls show a high rate of switching voters. While the 2017 election saw the Conservative and Labour parties winning 84 percent of the vote — a combined share they had not reached since the “good old days” of two-party politics — the latest polls suggest these two parties would now win just over 60 percent of the vote. The remaining 40 percent is taken up by the resurgent Liberal Democrats and a Brexit Party born from the ashes of the U.K. Independence Party.

Because voters are more willing to switch parties, they’re also likely to be more susceptible to political campaigning. In the 2017 election, the Conservatives had a disastrous campaign: Over six weeks, a 20-point lead dwindled to just 2 points. In Prime Minister Boris Johnson, the Conservatives have found a better campaigner than former Tory leader Theresa May. But as Johnson’s Withdrawal Agreement Bill comes under greater scrutiny in the weeks ahead, he’s likely to find he’s given the opposition plenty of ways in which to attack his plans.

If it is difficult to say how many votes each party will get on the day of the election, it is even more difficult to say where they will get those votes.

Britons elect their members of parliament in single-member districts where the candidate with the most votes wins the seat. A party can get a large share of the vote but win no seats because that vote is spread evenly across the whole country. This looks to be the fate of the Brexit Party, a party that is polling between 11 percent and 13 percent, but is unlikely to win a single seat.

This means that forecasting the distribution of votes is critically important to predicting the share of seats each party will get. But again, there are serious difficulties here.

For most of the post-war period, political scientists have assumed that constituencies behave roughly the same. When a party improves its vote share by 2 percent or 3 percent, its vote share goes up by 2 percent or 3 percent in every constituency. This is something called uniform national swing, or UNS.

UNS has often worked well, but it didn’t work well in 2017. In that election, the Conservatives did much better in areas that voted to leave the EU. Labour did much better in areas which voted to Remain. That trend might continue in 2019, as British party politics reconstitutes itself around Brexit — or maybe it’ll be back to business as usual.

The proliferation of polls during the campaign — and their effect on voter behavior — will add yet another complicating factor.

UK NATIONAL PARLIAMENT ELECTION POLL OF POLLS

For more polling data from across Europe visit POLITICO Poll of Polls.

Because more voters identify with their vote to Leave or Remain than with a political party — and because no single party captures all of the Leave or Remain vote — they are likely to vote “tactically,” and look for the party that stands the best chance of winning in their area.

Parties will attempt to capitalize on this. The Brexit Party will try to retake Leave voters from the Conservatives. The Liberal Democrats will try to convince Remain voters that Labour’s policy on Brexit merely mimics that of the Conservatives.

Voters tempted by these smaller parties will want to know whether they stand a realistic chance of winning their seat, or whether they should vote tactically for the second-best option.

Because the last election is a poor guide to what will happen now, seat-level polling or seat models can fill that gap. Some voters will lap up this new information. For others, the result of all this data and rhetoric will be confusion: Estimates of “tactical voting” in previous elections suggest that up to a third of voters make an “incorrect” tactical vote, by voting for their second-best party even when their preferred party stands a better chance of winning.

Far from providing the U.K. with a sense of stability, this new election will introduce yet another source of uncertainty into a political landscape already convulsed by Brexit.

Chris Hanretty is a professor of politics at Royal Holloway, University of London.