LONDON — LONDON (AP) — Keir Starmer isn’t on the ballot, but the U.K. prime minister’s future is on the line in a special election on Thursday.

Voters in the Makerfield district of northwest England are electing a new lawmaker, and the leading contender is Andy Burnham of the governing Labour Party, the current mayor of Greater Manchester and oddsmakers’ favorite to be the next prime minister.

If Burnham defeats a candidate from the anti-immigration party Reform UK and wins the seat for Labour, he’s almost certain to challenge the embattled Starmer for leadership of the party, and the country.

Burnham has pledged that “if people put their trust in me, I will change politics” — a big promise for a politician who, if he wins, will be just one of 650 lawmakers in the House of Commons.

But the scores of journalists from around the world who have flocked to Makerfield during the campaign are evidence that this is no normal by-election, the results of which are due early Friday.

About 75,000 people are eligible to vote in Makerfield, a constituency that encompasses several towns and villages on the edge of Greater Manchester, 200 miles (320 kilometers) northwest of London.

They hold in their hands the fate of Starmer, whose popularity has cratered since he led the center-left Labour Party to a landslide election victory in July 2024.

Starmer’s government has struggled to deliver promised economic growth, repair tattered public services and ease the cost of living, and been hamstrung by repeated missteps, including his decision to appoint Peter Mandelson, a scandal-tarnished friend of Jeffrey Epstein, as the U.K. ambassador to the United States.

A dismal performance in May’s local elections spurred scores of Labour lawmakers to demand Starmer’s resignation. He has refused to budge, but senior colleagues are trying to force a change. Wes Streeting resigned as health secretary in May, saying that “where we need vision, we have a vacuum.”

Then Josh Simons, the Labour lawmaker for Makerfield, stepped down to trigger a special election and give Burnham the chance to return to Parliament.

Britain’s parliamentary system allows governing parties to change leader midterm, with the winner becoming prime minister without the need for a national election. Under Labour rules, a lawmaker can challenge the leader if they have backing from a fifth of the party’s House of Commons lawmakers — a number that stands at 81.

Streeting said Tuesday that he hopes Starmer will agree to step down, but that if he doesn’t, “there will need to be a contest, and I would be prepared to do that.”

Streeting is an assured communicator with a base of support among parliamentary colleagues, but Burnham is considered the more likely successor.

The 56-year-old politician nicknamed the “King of the North” has led Manchester since 2017, overseeing rapid regeneration for the city where the Industrial Revolution was forged. Burnham is pledging to repeat his signature brand of “Manchesterism” on a national scale.

“It’s not right, the way the country has been run,” Burnham said on the campaign trail last week, claiming “London-centric politics” has failed other regions of the U.K.

Starmer, meanwhile, has tried to keep calm and carry on, insisting during a G7 summit in France this week that he has no intention of leaving his post.

“I will fight if there’s a challenge,” he said. “We won a significant general election result in 2024, with a mandate to bring about change. I’m not going to walk away from that.”

Starmer suggested that he could offer Burnham a Cabinet post if he wins, telling Sky News on Wednesday that “I want him to have a big role in government.” Allies of Burnham indicated that he wasn’t interested.

Rob Ford, professor of political science at the University of Manchester, said that if Burnham wins convincingly, “the pressure on Starmer will be very hard to resist.

“Starmer can say all that he likes that he wants to carry on,” Ford said. “But if the entire Cabinet turns around and says, ‘We’re not going to serve under you and we think you should go,’ then either he’ll go with dignity or go without dignity, but he’ll end up having to go quite quickly.”

Burnham’s victory isn’t a given. The Makerfield area has elected Labour lawmakers for more than a century, but Reform UK has rapidly gained ground in post-industrial northern England, winning big in local elections last month.

Reform candidate Rob Kenyon, a local plumber, is hoping to tap into concerns about immigration — frequently expressed by voters despite relatively low numbers of immigrants in Makerfield. But Reform faces a challenge from Restore, an even more hard-line, anti-immigration and ethnonationalist party to its right.

A Burnham victory would be bad news for Starmer. But Ford said that a Reform win in Makerfield would spell “Gotterdammerung, apocalypse, disaster, chaos” for the Labour Party.

“Andy Burnham is miles more popular than every other (leadership) candidate available. Miles better known, miles better liked,” he said.

“If Reform take him out, then simultaneously you have a situation where the Reform threat looks much graver, and the best person available to combat the Reform threat has failed.”