Birmingham, Alabama (CNN) Sen. Luther Strange and Roy Moore are into the final hours of a hard-fought intra-party campaign that is testing President Donald Trump's influence with Republican voters and could set the stage for a series of GOP Senate primaries next year.

The final day of campaigning here features Vice President Mike Pence as Strange's most visible advocate and Steve Bannon, the recently ousted former White House chief strategist, stumping for Moore.

Pence will visit NASA's Marshall Space Flight Center in Huntsville, and then head to Birmingham, Alabama's largest city, for a nighttime rally to close Strange's campaign.

Bannon, meanwhile, heads to Fairhope -- just outside Mobile, on the Gulf of Mexico -- to join "Duck Dynasty" star Phil Robertson at a Monday night "faith and family rally" with Moore.

Bannon will also appear on Fox News host Sean Hannity's show from the rally, giving Moore's side one final audience with Hannity's right-wing audience.

The last-minute campaigning comes amid deep fears among Republicans in Washington about what a Moore win could mean for the party in 2018's midterm elections.

Strange has been propped up by more than $9 million in spending from the Senate Leadership Fund, a super PAC aligned with Senate Majority Leader Mitch McConnell. Moore, meanwhile, has cast the campaign as an opportunity to reject McConnell -- a much less popular figure among Alabama Republicans than Trump.

Trump's visit to Alabama on Friday night was designed to lend his popularity to Strange, who Trump told a crowd of thousands has been a loyal ally in the fight to repeal former President Barack Obama's Affordable Care Act.

But the President acknowledged the potential that Moore could win -- and what such a victory would mean for Trump.

"If Luther doesn't win, they're not going to say we picked up 25 points in a very short period of time," Trump told the crowd. "They're going say, 'Donald Trump, the President of the United States, was unable to pull his candidate across the line.'"

Alabama's matchup is unusual in that many of Trump's core supporters also back Moore, the 70-year-old former state Supreme Court justice who was ousted from that job twice -- first in 2003 for refusing to remove a monument of the Ten Commandments at a state court building, then again recently for refusing to abide by the US Supreme Court's ruling legalizing same-sex marriage nationwide.

Moore's campaign even touted Trump's comment from his Friday night rally in Huntsville that he would "fight like hell" for Moore if he defeats Strange.

Either Strange or Moore would be favored against the Democratic nominee, Doug Jones, in the December special election for the seat formerly held by Attorney General Jeff Sessions, and which Strange has held since being appointed in February.

"If (Strange) loses, I think the reality is that the Republicans will still hold that seat," White House legislative director Marc Short said Sunday on NBC's "Meet the Press."

Still, a Moore win could be damaging for Republicans nationally. He is well-known for a history of bombastic statements about race sexual orientation and more -- such as Thursday night, when he told a debate audience that "abortion, sodomy, sexual perversion sweep our land." Democrats could use Moore's remarks as a fundraising tool and to put Republicans on the spot.

Republicans are closely watching Alabama for signs of what could happen in other races featuring anti-establishment challengers next year.

In Nevada, Republican Sen. Dean Heller already faces a primary fight against Danny Tarkanian. In Arizona, Sen. Jeff Flake faces conservative Kelli Ward. Both, like Moore, have Bannon's support. And across the board, Republican candidates recruited to take on Democratic incumbents in states won in 2016 face tough battles against farther-right foes.