President Trump's decision late Tuesday to endorse Sen. Luther Strange (R-Ala.) adds more uncertainty to the state’s fast-approaching special election.

The president’s shadow has loomed large over the Alabama GOP Senate primary. Strange and his two rivals, Rep. Mo Brooks Morris (Mo) Jackson BrooksOvernight Defense: Trump hosts Israel, UAE, Bahrain for historic signing l Air Force reveals it secretly built and flew new fighter jet l Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' OVERNIGHT ENERGY: House Democrats tee up vote on climate-focused energy bill next week | EPA reappoints controversial leader to air quality advisory committee | Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals,' official says MORE (Ala.) and former Alabama Supreme Court Chief Justice Roy Moore, are all jockeying for the “pro-Trump” mantle in a race that has turned ugly.

So with less than a week to go before Tuesday’s primary, Trump’s move will give Strange a boost as he looks to land a spot in an expected runoff.

“The current electorate is pretty much decided, and I don’t think the Trump endorsement changes a whole lot with those voters,” one unaffiliated Alabama Republican strategist told The Hill.

“But what it could potentially change is the turnout. There are voters out there who weren’t interested who now may become interested because of Trump’s endorsement.”

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Trump’s endorsement could resonate with GOP primary voters. Eight months after winning the GOP primary by 22 points, he won the state’s general election by 28 points.

Now he holds a 55 percent job approval rating statewide, according to a recent Gallup survey — and his approval rating is far higher among Republicans.

Trump’s endorsement comes after months of labor by top Republicans like Senate Majority Leader Mitch McConnell Addison (Mitch) Mitchell McConnellMomentum growing among Republicans for Supreme Court vote before Election Day Trump expects to nominate woman to replace Ginsburg next week Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral MORE (Ky.), who want the GOP to unite around Strange.

The National Republican Senatorial Committee (NRSC) considers Strange the incumbent, which comes with the threat of blacklisting any campaign vendor who works for his opponents. The Republican National Committee has pitched in with $350,000 in spending through the NRSC. And McConnell’s allied super PAC —the Senate Leadership Fund (SLF) —has spent millions to elect Strange.

Trump’s decision to fall in line with the party is the final piece in the puzzle that gives Strange, who was appointed to the seat vacated by Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsTrump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status White House officials voted by show of hands on 2018 family separations: report MORE in February, the backing of virtually every major Republican figure.

“Senator Luther Strange has done a great job representing the people of the Great State of Alabama. He has my complete and total endorsement!” Trump said on Twitter.

The SLF is highlighting that endorsement in a new digital advertising push meant to shore up the idea that Strange is the most pro-Trump candidate in the race.

Brooks responded with a strident statement Wednesday morning that argued that McConnell and the “swamp” are misleading “the president into endorsing Luther Strange.”

He accused Strange of “corruptly and unethically” bribing the former Alabama governor, Robert Bentley, into appointing him to his Senate seat. And he called Strange “Lyin’ Luther,” recalling Trump’s own penchant for pejorative nicknames.

As state attorney general, Strange had been involved in an investigation into Bentley, who later resigned and pleaded guilty in relation to an alleged cover-up of an affair. But there’s no evidence of any wrongdoing surrounding the appointment, which was made in the midst of that investigation.

The tone of Brooks’s statement underscores the brutal feud that’s developed between Strange and Brooks over Trump.

The SLF has led the charge on a multimillion-dollar onslaught of ads highlighting Brooks’s past criticisms of Trump during the GOP presidential primaries, where he called Trump a “serial adulterer” and questioned whether he’d follow through on lofty campaign promises.

Those ads have become ubiquitous in Alabama, with one even airing in the middle of a Brooks interview on a talk radio station last month.

Brooks has sought to push back against those attacks by arguing that he shouldn’t be judged for comments he made during the primaries, saying that he’s been a vocal supporter of Trump’s agenda since then.

But Brooks has also more recently shared criticisms of the president, particularly over inaction on curbing Obama-era immigration programs and for Trump’s decision to publicly criticize Sessions for recusing himself from the Justice Department’s investigation into Russian influence in the 2016 presidential campaign.

The endorsement comes at a critical time for Strange, with less than one week before the primary. If no candidate garners a majority of support on Tuesday, as is expected, the top two candidates move on to a runoff in September.

Most Alabama Republicans believe Moore is in the driver’s seat for a spot in the runoff, which would leave Strange and Brooks battling for the second spot.

The most recent public polling by JMC Analytics and Polling shows Moore with 30 percent support, followed by Strange at 22 percent and Brooks at 19 percent. Moore is the only candidate with positive favorables — both Strange and Brooks sit underwater by double-digit margins.

Despite the closer public polling, Strange allies remain confident that he is on a clear path to make the runoff.

“We feel good—we have always been trying to get Sen. Strange into the runoff and it looks like that will happen,” said SLF spokesman Chris Pack.

“The fact that President Trump endorsed Sen. Strange last night will be a big boost.”

But with public polling showing both Brooks and Strange’s favorable ratings underwater, some Republicans believe the ads have also taken their toll on Strange.

“Any time you or anyone on your behalf goes negative, you get the blame,” the Alabama GOP strategist said.

Bill Kling, a Huntsville city councilman who’s known Brooks for years but is remaining neutral, told The Hill he’d expect the negative attacks to depress turnout, which could cast more uncertainty on an already-difficult-to-predict special election primary.

While the feud between Strange and Brooks has dominated the headlines and the spending for the vast majority of the race, Moore has largely kept out of the fray and out of SLF’s spotlight despite his strong polling.

That changed last week, as the super PAC has begun to pivot to Moore, expecting the impending runoff between him and Strange. Trump’s endorsement marked the nail in the coffin as far as SLF’s advertising strategy was concerned — the endorsement prompted the group to replace its few remaining anti-Brooks ads with those attacking Moore.

In response, Moore responded to those ads with a spot of his own that calls on voters to “drain the swamp, send McConnell a message.” That ad needles Strange and McConnell as members of the “swamp” along with Democrats like House Minority Leader Nancy Pelosi (Calif.) and anti-Trump Republicans like Sen. Jeff Flake Jeffrey (Jeff) Lane FlakeJeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Republican former Michigan governor says he's voting for Biden Maybe they just don't like cowboys: The president is successful, some just don't like his style MORE (Ariz.).

Moore has a strong base of support in the state, thanks to his public defiance to a court order to remove a statue of the Ten Commandments he had commissioned for state grounds, which cost him his position in the state Supreme Court. He won his post back in 2012, but his refusal to follow the Supreme Court’s 2015 order legalizing gay marriage caused another suspension.

While Moore has targeted Strange, he’s left Brooks alone, presumably because of the assumption that Brooks’s supporters are more likely to flock to Moore if their pick falls short.

“Brooks and Moore will be perceived as having similar, more conservative beliefs where Strange might be more of a mainline conservative,” Kling said.