President Donald Trump urged voters on Monday to reject a maverick Senate candidate from his own camp, an ex-con coal baron who Republicans fear is too radical to prevail in November’s mid-term elections – and could cost them a winnable seat.

Proclaiming himself "Trumpier than Trump," Don Blankenship has mounted a highly controversial campaign ahead of Tuesday’s primary in West Virginia, setting off alarm bells among Republicans facing a stiff challenge to maintain control of the US Congress.

Mr Blankenship – a former coal executive who spent a year in prison over safety lapses linked to the deaths of 29 miners – has peddled conspiracy theories, used racially charged terms, and coined incendiary, Trump-like nicknames for his opponents.

On Monday, Trump used his Twitter bully pulpit to urge "the good people of West Virginia" to reject Mr Blankenship and instead pick one of two rival Republicans – congressman Evan Jenkins or state Attorney General Patrick Morrisey.

December’s Senate election in conservative Alabama proved disastrous for Republicans, when Roy Moore, who was endorsed by Trump despite accusations he once molested underage girls, lost to a Democrat.

Mr Moore was among a handful of extreme candidates in recent years who have won the Republican nomination only to eventually lose the Senate election.

The president and his party now appear alarmed that a primary win by Mr Blankenship would sink prospects for ousting Democratic incumbent Senator Joe Manchin – even in a state Mr Trump won by 42 points in 2016 – and make it harder for the party to hold its narrow 51-49 Senate majority.

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Republican groups have thrown big money at the race, releasing ads branding Mr Blankenship a "convicted criminal".

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Mr Blankenship, meanwhile, has grown increasingly defiant, using his fortune to blanket the state with ads attacking establishment leaders including Senate Majority Leader Mitch McConnell, who he has branded "Cocaine Mitch" – in reference to drugs once being found aboard a ship owned by his wife’s family.

"Tomorrow, West Virginia will send the swamp a message – no one, and I mean no one, will tell us how to vote," Mr Blankenship said in a Facebook post.

Several states hold Republican primaries on Tuesday for Senate and House contests, as the country enters a deeply divisive campaign season ahead of November mid-terms.

Among the congressional candidates in Indiana’s Republican primaries is Greg Pence, the older brother of US Vice President Mike Pence who held the seat for a decade.