Former Alabama chief justice Roy Moore has snatched a solid lead in the state's Republican run-off against interim incumbent Senator Luther Strange, the man Trump hopes will retain the seat vacated by now-Attorney General Jeff Sessions.
While the politics are local, the race is very much on the national radar, having emerged as a test of Trump's endorsement power -- and, if Moore wins, a potential shot across the bow of any Republicans too comfortable with the status quo in Congress.
Alabama has not elected a Democrat to the Senate in a quarter century, so whoever prevails in Tuesday's Republican run-off will likely win the December 12 general election and head to Washington.
Trump travels to the deep southern state Friday for a campaign rally with Strange, who trails Moore by nearly nine percentage points, according to a poll average compiled by RealClearPolitics.
In an awkward political split, former White House aide Sebastian Gorka and Trump supporter Sarah Palin, the 2008 Republican vice presidential candidate, join a Thursday rally for Moore.
Steve Bannon, Trump's controversy-fuelling former White House strategist, has also lined up behind the challenger.
Bannon and the rightwing Breitbart News website he heads are funded by hedge-fund billionaire Robert Mercer who, like some other wealthy conservatives, are waging a war of ideas against the Republican establishment.
"It's typical of Republican primary battles that we've seen over the past few years," Robert Boatright, a political science professor at Clark University, told AFP. "The candidate is almost incidental."
On Wednesday Trump said Strange would be a reliable Republican in the Senate, where a clash over health care has highlighted the intransigence of some in the president's party.
"I am supporting 'Big' Luther Strange because he was so loyal & helpful to me!" Trump tweeted.
Fealty is no motivator for Moore, who has branded his opponent an "establishment lackey" to Senate Majority Leader Mitch McConnell.
Moore, 70, routinely advocates a return to Christian religious values. When he was chief justice of the Alabama Supreme Court, he fought against efforts to remove a display of the Ten Commandments from the courtroom.
He was twice suspended from the court for ethics violations related to his order against issuing marriage licenses to same-sex couples.
'Loyal supporter' vs 'loose cannon'
Moore backs impeachment of US Supreme Court justices "if they put themselves above the Constitution," and he has come out strongly opposed to legalizing hundreds of thousands of young immigrants, known as "Dreamers" who were illegally brought to the United States by their parents.
"These are not dreamers, they are law breakers," Moore said recently on Twitter, in an apparent break from the president who has signalled he could strike a deal on letting Dreamers stay.
Trump won Alabama by 28 points last year. While his approval rating percentage has dwelled in the thirties since mid-May, there are large pockets of Republicans still deeply loyal to him.
Moore apparently is counting on a large evangelical base of support, and intense suspicion of Washington, to carry him through.
"He's defied the federal government, and Alabamians don't really like federal intrusion, especially these cultural issues like same sex marriage," said University of Alabama political science professor Carol Cassel.
Strange has his own political baggage. He was appointed to the Senate in February by Alabama's then-governor Robert Bentley.
That appointment became Strange's liability months later, when Bentley resigned over a sex-tainted scandal. Accusations swirled that Strange sought the Senate seat from Bentley while the governor was under investigation by Strange's office.
Strange has campaigned largely on economic issues, small government and gun rights, while Moore has focused "on religious and cultural issues," Cassel said.
"Strange is just seen as a loyal supporter, and Moore is more a loose cannon," she added.
For Clark University's Boatright, who predicts a Moore victory, one question is whether Trump will have sway over Alabama voters.
"The only real truth we'll see is Trump has not shown the ability to help anybody," he predicted.
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