Michigan’s Republican primary for governor has become unexpectedly unsettled despite Congressman John James entering as the clear favourite. The contest matters because it will decide who challenges the Democratic nominee to replace term-limited Governor Gretchen Whitmer, in a state viewed as important to both parties’ wider midterm prospects.

James has President Donald Trump’s endorsement and former attorney general Mike Cox withdrew and backed him, leaving businessman Perry Johnson as his principal active rival. However, party figures have criticised James’s campaign and questioned his general-election strength, while Johnson has spent heavily attacking him; James’s campaign says he remains the frontrunner and the Republican best placed to win in November.