LONDON — British Prime Minister Boris Johnson and opposition leaders were pushing for the finish line in Britain’s election campaign today, dashing through multiple constituencies to drum up support in the final 72 hours before polling day.

Johnson was touring Labour-held seats across England that his Conservatives have to win if they are to secure a majority in Thursday’s election. Labour Party leader Jeremy Corbyn was in southwest and central England, where his left-of-center party is trying to hang on to key constituencies.

Opinion polls give Johnson’s Conservatives a lead, but as many as one in five voters remain undecided. This election is especially unpredictable because the question of Brexit cuts across traditional party loyalties.

Underscoring the significance of those shifting allegiances, Johnson’s first stop was in the eastern port town of Grimsby targeting Labour voters who also backed leaving the European Union in the U.K.’s 2016 referendum.

Wearing a white jacket, hat and rubber boots, Johnson lugged a crate of fish across the floor of a fish market in the town that has been in Labour Party hands for 74 years — and said he was “taking nothing for granted” in the final days of campaigning.

The Conservatives had a minority government before the election, and Johnson pushed for the vote, which is taking place more than two years early, in hopes of winning a majority of the 650 House of Commons seats and breaking Britain’s political impasse over Brexit. He says that if the Conservatives win a majority, he will get Parliament to ratify his Brexit divorce deal and take the U.K. out of the EU by the current Jan. 31 deadline.