Bestselling author JD Vance has won Ohio’s contentious and hyper-competitive Republican Senate primary, buoyed by Donald Trump’s endorsement in a race widely seen as an early test of the former president’s hold on his party as the midterm season kicks into high gear.

Vance’s win brings to a close an exceptionally bitter and expensive primary contest that, at one point, saw two candidates nearly come to blows on a debate stage.

And it marks a major victory for Trump, who has staked his reputation as a Republican kingmaker on his ability to mobilise his supporters as he eyes another White House run in 2024.

Vance’s book “Hillbilly Elegy” describing the social problems and culture crisis in middle America became an international bestseller and was promoted as giving an insight into the conditions that led to Trump’s rise.

The author and venture capitalist had been behind in the polls before Trump waded into the race less than three weeks ago, endorsing Vance despite his history as a staunch Trump critic.

In 2016 Vance said the celebrity businessman could become “America’s Hitler” but has since become an avid supporter.

The author has said he was wrong and, like most of his rivals, tied himself to the former president, eagerly courting his endorsement and running on his “America First” platform, underscoring the extent to which the GOP race has transformed in his image.

Vance wooed the former president by echoing his bashing of immigrants, scepticism about US military involvement overseas — even in support of Ukraine — and lies about Trump’s defeat in the 2020 election.

He will face Democrat Tim Ryan, the 10-term Democratic congressman who easily won his three-way primary Tuesday night.

But November’s general election to fill the seat being vacated by retiring Republican Sen. Rob Portman is expected to be an uphill climb for Ryan in a state Trump won twice by an 8-point margin and in what is expected to be a brutal election year for Democrats trying to hold their congressional majorities