Donald Trump's visit to Alabama on Friday night to rally the troops for Republican Sen. Luther Strange figures to be a jubilant homecoming, as the state served as something of a launching pad for his presidential campaign.

But by Wednesday morning, he could be feeling a bit of a hangover.

A Republican primary runoff to fill the U.S. Senate seat held by now-Attorney General Jeff Sessions might be headed for an upset, as controversial Judge Roy Moore leads in the polls against Strange, who was appointed to the upper chamber in February.

A Moore victory would deliver a gut punch to Republican Party leaders, who have poured money in to defeat him and prop up the incumbent. Such an outcome is one Trump would understand better than anyone. Which is why voters in the state and conservative activists outside it were perplexed -- some even disappointed — that he waded into the race on behalf of the Mitch McConnell-favored candidate, or "Big Luther," as the president calls the 6-foot-9 senator.

Indeed, many of the president's supporters, including former adviser Steve Bannon, are backing Moore. The pro-Trump super Pac Great American Alliance hosted a rally on behalf of the former state Supreme Court chief justice Thursday night, where Sarah Palin, Texas Rep. Louie Gohmert, and recently ousted White House strategist Sebastian Gorka were headliners. Iowa Rep. Steve King endorsed Moore on Thursday.

Candidate Trump, campaigning in Alabama and running against the establishment GOP, might have also supported Moore, who is more in his mold than Strange. But President Trump doesn't have many loyalists in Congress, and insists he is rewarding the incumbent for being one of the few. "Looking forward to Friday night in the Great State of Alabama. I am supporting ‘Big’ Luther Strange because he was so loyal & helpful to me!" he tweeted.

Now, the president's task is to push Strange over the finish line in a race that places Trump himself on the other side of his supporters. And while Trump remains wildly popular in Alabama, the runoff on Tuesday will test his clout.

The administration is sending in the cavalry in the final days of the primary. Fresh off high-stakes meetings with world leaders at the United Nations and amid increased provocations from North Korea and a decision looming on the Iran nuclear deal, the president will travel to Huntsville Friday night to host a rally on behalf of Strange. On Monday night, Vice President Mike Pence will visit the state to campaign for the senator.

Trump's visit is likely to draw large crowds of excited fans. But Republicans in the state say such enthusiasm isn’t necessarily transferable.

"People are going to stick by the president regardless of who he is supporting. But it doesn't mean they're with Luther Strange or against Judge Moore," says GOP operative David Ferguson, an Alabama native aiding the administration ahead of the visit. "Alabamans don't necessarily want someone else to influence how they vote."

Strategists in the state say there is a deep distrust of Washington, and financial support directed by establishment-linked groups, like the McConnell-backed Senate Leadership Fund, might push Trump supporters away from Strange.

"It's Moore's race to lose at this point," says Alabama GOP strategist Brent Buchanan, noting outside private polling that finds Moore leading outside the margin of error. "Polling shows that Trump voters are voting for Moore."

In an attempt to overcome that disadvantage, Strange has endeared himself to Trump and makes virtually no mention of McConnell. He has positioned himself as the president's loyal lieutenant in the Senate, touting his votes in line with the administration. In ads, he laments "Washington politicians" and says he's "fighting to pass our president's agenda." The Senate Leadership Fund has spent $9 million on the primary, magnifying the smallest bits of daylight between Moore and Trump and highlighting the challenger's controversies.

Moore was removed as state chief justice in 2003 after refusing an order to remove a Ten Commandments monument from the state's high court building. He was later re-elected, but suspended in 2016 for telling Alabama judges to ignore the U.S. Supreme Court decision legalizing gay marriage. In 2005, he said homosexuality should be illegal, according to audio recently unearthed by CNN. And at a recent campaign stop, he referred to Native Americans and Asians as "reds and yellows."

Republicans in Washington are concerned about Moore joining the Senate, and whether that might inspire more insurgent candidates in GOP primaries. For his part, Moore has played up Strange's ties to Washington and financial support from "the swamp."

"Will McConnell's forces be able to control the senators coming up, with their money, their millions of dollars of money, in the Senate Leadership Fund?" Moore said at a recent campaign stop, according to a published report. In an early primary ad he sponsored, the narrator urges voters to "Drain the Swamp. Send McConnell a message. Send them all a message" as images include Strange's face along with those of Republican and Democratic leaders.

Strange comes with some baggage of his own that strategists say has had an impact on the race. The former state attorney general was appointed to the post by disgraced former Gov. Robert Bentley, who resigned as part of a plea deal after a sex scandal that evolved into other violations. Strange's office had been investigating some of Bentley's actions.

"The biggest thing Luther Strange is dealing with is how he got this appointment," says Birmingham-based GOP strategist Chris Brown.

Moore and Strange advanced to the runoff after an August primary that included Rep. Mo Brooks. Moore pulled in 40 percent of the vote and Strange took 32 percent. Brooks, who garnered 20 percent, has endorsed Moore.

Other top supporters of Trump, including Fox News Host Sean Hannity and former Arkansas Gov. Mike Huckabee, whose daughter is the White House press secretary, have also endorsed Moore. (Senior Alabama Sen. Richard Shelby is backing Strange, who has also secured an endorsement from the National Rifle Association.)

Yet Bannon's support of the challenger is among the most intriguing. Now back at the helm of Breitbart News, Bannon is reportedly directing negative coverage of Strange. On Thursday, the site went so far as to publish a story questioning whether Trump would draw the large crowds he is accustomed to in Alabama. A Moore victory could help fuel Bannon's mission to challenge other incumbents. A defeat could help diminish his influence outside the White House.

Upstarts such as Moore "are the types of candidates much more in line with the Trump agenda. It's going to be the grassroots against the establishment," says Eric Beach, a GOP operative who runs the Great America Alliance. The group sponsored the pro-Moore rally Thursday night, and recently added former Trump White House aide Andrew Surabian as a senior adviser. The group is also backing Kelli Ward, who is challenging Republican Sen. Jeff Flake in Arizona.

This dynamic reflects a notable GOP divide in the Trump era. A new Wall Street Journal/NBC News poll shows that 38 percent of Republican voters identify as supporters of the overall party, while 58 percent say they consider themselves more as supporters of Trump. The survey also found a three percentage-point increase in the president's approval rating, now at 43 percent. (Trump’s approval is 41.2 percent in the RCP average.)

Beach says there is a frustration among the Trump base that Congress is impeding the president's agenda, and he hopes Tuesday's election sends a message directly to GOP leadership.

"They need to understand the new Republican Party and follow the grassroots," Beach says.