(Reuters) - U.S. President Donald Trump is due to travel to Alabama on Friday to whip up votes in a Senate race that has laid bare the rift between the Republican party’s leadership and its anti-establishment wing.

Ex-Trump adviser Sebastian Gorka (L) applauds as he campaigns for U.S. Senate candidate Judge Roy Moore with his wife Kayla Moore (R) at the Historic Union Station Train Shed in Montgomery, Alabama, U.S., September 21, 2017. REUTERS/Tami Chappell

In a move that surprised political analysts, Trump will appear alongside the party-approved incumbent rather than the controversial insurgent.

The president’s decision to campaign for Senator Luther Strange ahead of Tuesday’s primary runoff election came despite Trump’s strained relationship with Republican Senate Majority Leader Mitch McConnell, one of Strange’s strongest supporters.

In last month’s initial round of voting, Strange finished second to former Alabama Chief Justice Roy Moore, whose “drain the swamp” credentials have been burnished by high-profile endorsements, including that of former White House strategist Steve Bannon.

“I think that if you were to approach this without any prior knowledge ... Judge Moore would be the more logical ‘Trumpian’ candidate,” said Steven Taylor, a political science professor at Troy University in Alabama.

The winner on Tuesday will be running in December against Democratic candidate Doug Jones in the Republican-dominated state.

Neither Republican campaign responded to requests for comment.

Moore, 70, is a religious conservative who twice lost his position as the state’s top judge. He was ousted in 2003 after he refused a federal court order to remove a Ten Commandments monument from the state Supreme Court building.

After winning back the seat in 2013, he directed state judges to ignore the U.S. Supreme Court’s ruling legalizing same-sex marriage, was suspended and then resigned.

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Moore has vowed to support Trump’s agenda and has blamed McConnell for the Senate’s lack of major legislative victories.

A Moore campaign video released last week showed a photo of McConnell beside the words “Washington insiders” as Moore’s wife warned he would not “bend his knee.”

Strange, 64, was appointed to fill the Senate seat left vacant when Jeff Sessions became U.S. attorney general.

Known as “Big Luther” due to his 6-foot-9-inch stature, he has been backed by nearly $9 million of advertising from a McConnell-allied political action committee.

“I am supporting ‘Big’ Luther Strange because he was so loyal & helpful to me!” Trump tweeted on Wednesday.

Strange served as state attorney general under former Governor Robert Bentley, who resigned in April.

Moore has endured his share of controversy, including negative press this week after he mentioned “reds” and “yellows” in an apparent reference to American Indians and Asians.

In a recent advertisement, Strange questioned Moore’s commitment to Trump’s planned wall at the Mexican border.

“Roy Moore: wrong on President Trump, wrong on the wall, wrong for Alabama,” the ad’s narrator says.

The two candidates are scheduled to square off at their only debate on Thursday.

With turnout expected to be low for Tuesday’s runoff, Troy University’s Taylor said both campaigns have reason to believe they hold an edge.

“Moore’s advantage is that he does appeal to the true-believing base,” Taylor said. “In terms of which of the campaigns is going to be more adept at the ground game, I would think Strange has an advantage.”