Senate Majority Leader Mitch McConnell mocked former Donald Trump adviser Steve Bannon for promoting a Republican candidate who lost in deeply conservative Alabama.

“The political genius on display, throwing away a seat in the reddest state in America, is hard to ignore,” Mr McConnell told reporters. The unlikely victory by Democrat Doug Jones shaved Mr McConnell's already-tenuous majority in the Senate to a single vote.

Formerly a top campaign adviser and aide to Mr Trump, Mr Bannon left the White House and has continued to promote a nationalist agenda from atop the website Breitbart — an agenda that often clashes with the views of a Republican leadership Mr Bannon openly despises. He has said it is a “season of war against a GOP establishment”.

The Alabama Senate race put those fractures in the Republican Party on vivid display. Mr Bannon championed far-right candidate Roy Moore throughout the race, breaking with establishment Republicans like Mr McConnell.

Voters sided with Mr Bannon during the primary, elevating Mr Moore over the more centrist Luther Strange, who drew the backing of party grandees including Mr Trump himself.

Mr Moore was already problematic for some Republicans because of his controversial history: he was twice removed from the Alabama Supreme Court for defying the law, said a Muslim person should not be allowed to serve in Congress and called for homosexuality to be outlawed.

But he became toxic to Mr McConnell and others in the party leadership after allegations emerged he had engaged in sexual misconduct with teenage girls (Mr Moore denied the accusations, saying “I have never engaged in sexual misconduct”).

Alabama Senate race: Roy Moore vs Doug Jones Show all 12 1 /12 Alabama Senate race: Roy Moore vs Doug Jones Alabama Senate race: Roy Moore vs Doug Jones Democratic senatorial candidate Doug Jones speaks to reporters after voting Getty Images Alabama Senate race: Roy Moore vs Doug Jones Republican Senatorial candidate Roy Moore (L) and his wife Kayla ride their horses to the polling station to vote AFP/Getty Images Alabama Senate race: Roy Moore vs Doug Jones David Watson waves in a Santa suit as he holds a Doug Jones sign AP Alabama Senate race: Roy Moore vs Doug Jones Republican Senatorial candidate Roy Moore speaks to the media as his wife Kayla (R) looks on AFP/Getty Images Alabama Senate race: Roy Moore vs Doug Jones Democratic Alabama Senate candidate Doug Jones casts his vote at Brookwood Baptist Church REUTERS Alabama Senate race: Roy Moore vs Doug Jones Roy Moore speaks to the media after he rode in on a horse to vote AP Alabama Senate race: Roy Moore vs Doug Jones Democratic candidate Doug Jones greets supporters after casting his ballot AP Alabama Senate race: Roy Moore vs Doug Jones Republican Senate candidate Roy Moore departs on horseback after he cast his ballot in Gallant REUTERS Alabama Senate race: Roy Moore vs Doug Jones Voters wait in line to cast their ballot at a polling station setup in the St Thomas Episcopal Church Getty Images Alabama Senate race: Roy Moore vs Doug Jones Democratic Senatorial candidate Doug Jones (L) greets voters outside of a polling station at the Bessemer Civic Center Getty Images Alabama Senate race: Roy Moore vs Doug Jones Voters exit after casting their ballots at a polling station setup in the Fire Department Getty Images Alabama Senate race: Roy Moore vs Doug Jones Democratic Alabama U.S. Senate candidate Doug Jones speaks with the media after casting his vote at Brookwood Baptist Church Reuters

But Mr Bannon’s website, Breitbart, dispatched journalists to try and discredit reporting around those allegations, and Mr Bannon himself continued campaigning with Mr Moore until the end, when voters rejected his chosen candidate.

Soon after those results came in, verifying that Republicans had surrendered a seat in a state where victories are almost automatic, allies of Mr McConnell began blaming Mr Bannon. The head of a McConnell-aligned political committee that seeks to elect Senate Republicans released a statement that evening blasting Mr Bannon.

“Not only did Steve Bannon cost us a critical Senate seat in one of the most Republican states in the country, but he also dragged the President of the United States into his fiasco,” said Senate Leadership Fund CEO Steven Law, who served as Mr McConnell’s chief of staff.