(CNN) President Donald Trump is set to step into a Senate race in Alabama that will test whether his word is enough to sway Republican voters in a hard-fought Bible Belt contest.

Trump will campaign Friday night in Huntsville alongside Sen. Luther Strange -- a recent appointee who has based his entire campaign on his allegiance with the President.

Strange faces Roy Moore , the twice-ousted former state Supreme Court chief justice, in a Republican primary runoff Tuesday for the seat previously held by Attorney General Jeff Sessions.

Trump's popularity is Strange's strongest asset against Moore, who has over decades developed a loyal following in Alabama based on his message of making Christianity prominent in public policy-making and restricting LGBT rights.

In a debate against Moore on Thursday night, Strange boasted that he'd spent 30 minutes on the phone with Trump the previous night.

"We've developed a close, personal friendship. We both come from the same background, the same mission, the same motivation to make this country great again," Strange said.

"We've sort of bonded," he said. "I've not been in Washington as long as the President has. He's learned the ways of Washington the hard way -- lots of criticism, lots of people standing in the way -- and so have I."

Trump also bragged Thursday on Twitter of the importance of his support for Strange.

"Senator Luther Strange has gone up a lot in the polls since I endorsed him a month ago. Now a close runoff. He will be great in D.C.," Trump tweeted.

Senator Luther Strange has gone up a lot in the polls since I endorsed him a month ago. Now a close runoff. He will be great in D.C. — Donald J. Trump (@realDonaldTrump) September 21, 2017

He reiterated his boast Friday: "Will be in Alabama tonight. Luther Strange has gained mightily since my endorsement, but will be very close. He loves Alabama, and so do I!"

Will be in Alabama tonight. Luther Strange has gained mightily since my endorsement, but will be very close. He loves Alabama, and so do I! — Donald J. Trump (@realDonaldTrump) September 22, 2017

Moore, meanwhile, hit Strange for being slow to support Trump's calls for the elimination of the Senate rules requiring 60 votes for legislation to move forward -- a threshold that has given Republicans, who hold 52 seats, no way to advance some bills without Democratic support.

He said he'd more effectively carry out Trump's agenda, even without a personal relationship.

"I can't tell you what the President thinks. I can't tell you every move he makes -- when he goes to the bathroom, when he doesn't," Moore said.

At left, Sen. Luther Strange, and on right, Roy Moore, both Republicans from Alabama

Alabama race will impact Republicans nationally

For the Republican Party nationally, though, the contours of the contest are far less important than what a Moore victory could mean for the party's brand.

In comments that underscored some within the GOP's fears that Moore could cost the party support, particularly outside the Bible Belt, Moore warned Thursday night that "transgender troops in our bathrooms and inclusiveness" have harmed the military and declared that "God is the only source of our law."

"Our foundation has been shaken," Moore said. "Crime, corruption, immorality, abortion, sodomy, sexual perversion sweep our land."

The religious focus and anti-LGBT message is at the core of Moore's political identity. He was booted as chief justice first in 2003 for refusing to remove a monument of the Ten Commandments from a state court building, and again in 2016 for ignoring the US Supreme Court's ruling legalizing same-sex marriage

Moore bashed Strange for following that high court ruling as Alabama's attorney general. "He caved. He did not stand," Moore said.

Strange, meanwhile, boasted of defending "our precious religious liberty" by litigating on behalf of a Christian broadcasting company that opposed Obamacare's mandate that insurance plans cover contraception.

Moore, widely known across Alabama, finished first in an August primary with 39% of the vote -- 25,000 votes ahead of Strange. Since then, he's won the endorsement of Rep. Mo Brooks, the third-place finisher, whose strength in his northern Alabama district makes places like Huntsville, where Trump will visit Friday and Vice President Mike Pence has planned a stop Monday, a battleground.

Trump considered leaving Strange on his own in the runoff against Moore as Moore surged ahead in late-August polling, CNN reported Thursday

But top White House staffer Rick Dearborn, a former Sessions chief of staff, persuaded Trump that Strange's support on immigration and budget issues and the potential that a Senator Moore could become a nuisance meant sticking with Strange was in Trump's interests.

Tennessee Sen. Bob Corker later visited the White House and told Trump that supporting Strange publicly would send a signal to future endangered GOP incumbents that Trump could boost their political standing. And Trump was shown polling indicating that the race had tightened.

The role Mitch McConnell is playing

The race is the first of several tests for Republican senators who are facing primary contests -- and where the challenger has made opposition to McConnell a centerpiece of his campaign.

Spending in the race has far exceeded most primaries, with the Mitch McConnell-aligned Senate Leadership Fund super PAC pumping $8 million into the effort to boost Strange and diminish Moore.

Moore said the Senate majority leader is to blame for the failure, thus far, to repeal former President Barack Obama's signature health care law.

"President Trump's being cut off in his office. He's being redirected by people like McConnell who do not support his agenda -- who will not support his agenda in the future," he said.

Strange shot back that to suggest Trump "is being manipulated by Mitch McConnell is insulting to the President."

The race has, for the first time, pitted Trump against his former chief strategist Steve Bannon , who has urged conservatives to support Moore and used Breitbart.com, where he is an executive, to bash Strange.

Strange alluded to Bannon's support for Moore on the debate stage Thursday.

"Many of the people who are supporting you look like the unemployment line at the White House," he said. "They were fired."

Among Moore's most frequent attacks on Strange has been how he got the job: Strange was appointed in February to fill Sessions' vacancy by former Alabama Gov. Robert Bentley, a Republican who shortly afterward left office amid a sex scandal . Moore asked Thursday whether Strange, as attorney general, had been investigating Bentley at the time of his appointment -- but Strange did not answer.

As the two closed their moderator-free debate Thursday, Moore said that "there is a God in Heaven that's in this campaign."

Strange shot back that he believes God is on both sides of the contest. But, he added: "The President is on my side."