MANCHESTER, England, Feb 26 (Reuters) - Keir Starmer faces a big test on Thursday when voters in Manchester cast their ballots for a new lawmaker in an election polls say is too close to call between the British leader’s Labour Party, populist Reform UK and the left-leaning Greens.

The vote in Gorton and Denton, in northwestern England’s Greater Manchester, is the latest challenge for Starmer after his judgment came into question over several policy U-turns and the appointment of Labour veteran Peter Mandelson as ambassador to Washington.

Making a surprise appearance in the area on Monday, Starmer and his team have tried to ram home the message that the so-called by-election is a simple choice between Labour and Reform, which is led by Brexit figurehead Nigel Farage.

They have repeatedly said a vote for the Greens, which could steal votes with its left-leaning policies on tackling the cost of living and its support for Gaza, is “a vote for Reform”.

ELECTION FOLLOWS SERIES OF MISSTEPS

The choice “could not be more stark”, Starmer said in a statement ahead of the vote. “Driving down the cost of living with Labour or driving a wedge between communities under Reform. Moving forwards together, or opening up anger and division that holds our country back.”

Labour won the seat easily in its landslide general election victory in 2024, but the incumbent lawmaker said last month he would step down.

With polls suggesting the three parties will win around the same vote share, the ballot is the first electoral test for Starmer since he came under fire for appointing Mandelson, a man whose close ties to the late U.S. sex offender Jeffrey Epstein have come into full focus, to Britain’s most prominent ambassadorial post.

Mandelson denies any wrongdoing.

That appointment added to a list of missteps and policy U-turns that have not only raised questions over Starmer’s judgment but also triggered some in his party to call for his resignation.

The 63-year-old has vowed to fight on, but in what could be an added disadvantage in Gorton and Denton, he was blamed last month for blocking popular Manchester mayor, Andy Burnham, from becoming Labour’s candidate in the area.

Instead, Labour put forward Angeliki Stogia, a local councillor, to run against Reform’s Matt Goodwin, an academic and author, and the Green Party’s Hannah Spencer, another local councillor, in a campaign marked by accusations of foul play.

Losing the by-election in areas which have returned Labour politicians to Westminster for generations would be another blow for Starmer, especially if the Greens win.

But it might not be fatal, Labour lawmakers say. They instead point to May as the major test, when local elections for several English councils and the Welsh and Scottish assemblies take place.

(Reporting and writing by Elizabeth Piper; Editing by Aidan Lewis)