A parliamentary by-election in Gorton and Denton, outside Manchester, will test support for Britain’s prime minister at a moment of intense political pressure.
The arrest of the British former envoy to Washington, long a key figure in the Labour Party, deepened a scandal that has led to calls for Prime Minister Keir Starmer’s resignation.
Peter Mandelson, left, with Prime Minister Keir Starmer in Washington last year. Mr. Starmer fired Mr. Mandelson after learning of Mr. Mandelson’s friendship with the sex offender Jeffrey Epstein.