KIRKBY-IN-ASHFIELD, England — The red brick houses that march up a gentle rise in this worse-for-wear town have long stood as part of the “red wall” in British politics, the gritty stronghold of coal and factory towns in the Midlands and north of England that voted as reliably for the Labour Party as blue-collar precincts in the American Midwest once voted for the Democrats.

So when Natalie Fleet, the Labour candidate for Parliament, knocked on the first of those doors last week, and was told by the woman who answered, Donna Savage, that she was thinking of voting for the Conservatives in next month’s election, it was a jolting sign of how much British politics has changed.

“I want to get Brexit done,” Ms. Savage, a 43-year-old teacher and lifelong Labour voter, told Ms. Fleet, echoing a phrase frequently used by the Conservative prime minister, Boris Johnson. Plus, Ms. Savage added, “I don’t want Jeremy Corbyn to be prime minister,” referring to the leftist Labour leader, who is deeply unpopular in this part of the country.