It wasn't until White House aides and campaign staffers convinced Trump that Strange could win that he went all in.
Trump had first endorsed "Big Luther" -- a nickname for the 6'9" former college basketball player -- in early August after his campaign coordinated with the White House for Trump's support.
"Senator Luther Strange has done a great job representing the people of the Great State of Alabama," Trump tweeted. "He has my complete and total endorsement!"
Strange, appointed to the seat formerly held by Attorney General Jeff Sessions, is seen as the GOP establishment candidate in the race. He enjoys the backing of Senate Majority Leader Mitch McConnell and has benefited from millions of dollars in ads from a super PAC aligned with the Kentucky senator. He faces off against Roy Moore, a candidate with a contentious past who is backed by former White House chief strategist Steve Bannon.
Moore has been removed from office twice; once for defying a court order to remove a monument to the Ten Commandments from the courthouse when he was the state's chief justice and another time for refusing to comply with a US Supreme Court decision legalizing same-sex marriage in 2016.
Not only could the outcome of the race presage a bitter primary season for Republicans against grassroots candidates in the 2018 midterm elections, it is also the first time since Bannon left the White House that he and Trump have been on opposing sides. A win for Moore would be a win for anti-establishment conservatives, an agenda Bannon advocated for when he was in the West Wing. Unlike the 2016 presidential election, Trump is on the establishment side this time.
Seen as the disruptive front-runner in the race, Moore's tendentious past has received a bevy of attention lately. CNN's investigative team KFILE unearthed a 2005 interview where he said that "homosexual conduct should be illegal."
Then, a decade later, his foundation -- "The Foundation for Moral Law" -- shared a video on its Facebook page that used out-of-context video clips
of then-President Barack Obama to promote the conspiracy theory that he is a Muslim, KFILE reported.
When contacted about the video, Moore told CNN's KFILE that it did not reflect his views and that his "campaign has nothing to do with Obama." However, as recently as December 2016, Moore said it is his "personal belief
" that Obama wasn't a natural-born citizen of the United States.
Strange finished second in the August primary -- 25,000 votes behind Moore, who secured 39% of the vote. No current polling for the primary race meets CNN's polling standards.
After Trump initially endorsed Strange, he was shown polling in late August that predicted Moore would defeat Strange by a significant margin, sources familiar told CNN. Trump weighed turning his attention away from the race, the sources said, and at that time there was no specific plan for him to visit the state to rally support for Strange.
Before making a final decision, Trump consulted with Rick Dearborn, a top White House staffer with deep ties to Alabama, to further explore Strange's chances. Though Dearborn is from Oklahoma, he was Jeff Session's chief of staff in his Senate office and is now a deputy chief of staff in the White House. Dearborn's relationship with Strange spans decades, and he's become a liaison between the White House and Strange.
Dearborn got in touch with Strange's campaign after speaking with the President. A source familiar with the conversation said the campaign implored Dearborn for the President's full endorsement -- reminding him of Strange's commitment to Trump's agenda on immigration and the budget, and arguing that a Senator Moore could be a White House nuisance. They also played up how both Trump and Strange are newcomers to politics.
In an interview with a local Fox station Thursday, Strange said he had developed a "personal friendship of trust and respect" with Trump.
"I respect his background, what he's trying to do, the challenges he's taking on," Strange said. "I think he recognizes that in my background. ... I think we have a personal relationship and he needs a partner in Washington desperately."
The White House was also told about a critical ad against Moore that the Strange campaign had in its arsenal. The White House said this was not part of the pitch to have Trump reinforce his endorsement, but sources familiar with the ongoing conversations said that's when he was informed of the ad. (Strange's campaign decided against running the ad.)
Senators have lobbed the president on Strange's behalf, too. At the end of a nearly 75-minute meeting with Bob Corker of Tennessee in the Oval Office last Friday, Trump asked Corker when they were one-on-one what he thought of the race, two sources familiar with the conversation said. Corker urged the President to visit Alabama, and added that his public support of Strange was not only critical for this race, but would signal to future endangered GOP incumbents that Trump could bolster support.
As others on Capitol Hill and campaign aides lobbied the President to become more involved in supporting Strange, Trump was shown a combination of external and internal polling that signaled the margin between Strange and Moore was tighter than previously imagined, according to one source inside the White House and two additional sources outside the White House.
Trump -- who easily won Alabama in the presidential election -- was ultimately convinced his support could help push Strange over the finish line.
The White House declined to comment on the process, stating the press office "cannot comment on political elections."
The President has since put significant muscle behind his endorsement. He will travel to the state Friday for a rally with Strange at the Von Braun Center in Huntsville, before Vice President Mike Pence does the same in Birmingham Monday.
Trump has also been effusive in his praise of Strange on one of his most popular platforms -- Twitter. In recent days, Trump has remarked that "Alabama is sooo lucky to have a candidate like" Strange and has commended him for being "tough on crime, borders and trade."
He also noted that, because the National Rifle Associated had endorsed Strange, "all gun owners should vote for Big Luther."
"He won't let you down!" Trump said.