With voting underway in Mississippi’s Senate runoff race, the big question for the final election of the 2018 midterm season revolves around a candidate who is not even on the ballot anymore.

Chris McDaniel, a firebrand Republican state senator, sought to upset the state's GOP establishment when he ran for the U.S. Senate seat held by Cindy Hyde-Smith. The Republican incumbent – appointed in March to fill a vacancy – came out on top in the four-way Nov. 6 election, but McDaniel was able to earn enough votes that no candidate secured a 50 percent majority to win the race outright, forcing Tuesday's runoff.

Now, his supporters are a key factor as Hyde-Smith looks to lock down the seat in Tuesday's runoff against Democratic former Secretary of Agriculture Mike Espy.

Thanks to a series of high profile flubs by Hyde-Smith and a huge push to boost turnout among Democrats, the race promises to be the most competitive Senate race the state has seen in decades. Unclear is whether McDaniel’s conservative supporters will show up to the polls on Tuesday or stay home. But if even a slice of the voters who went for McDaniel -- 17 percent of the Nov. 6 vote -- back Hyde-Smith on Tuesday, the incumbent might not have so tough a time.

“There has been a lot work done to make sure that McDaniel’s supporters will turn out, but the hardest part about runoff elections is getting people to turn out,” Jennifer Duffy, the senior editor for the Cook Political Report, told Fox News. “If they don’t turn out, McDaniel will be seen as the spoiler because if it wasn’t for him, there would not have been a runoff in the first place.”

McDaniel blasted Hyde-Smith during the primary campaign – accusing her of being a secret Democrat and not endorsing President Trump’s agenda. But the state lawmaker changed his tune following his defeat in early November.

“Mr. Espy cannot be allowed to win this seat,” McDaniel said shortly after conceding the race, according to Mississippi Today. “President Trump wants us to unite, and we will unite. We will back Cindy Hyde-Smith.”

McDaniel added: “Now I don’t agree with her. I don’t believe she’s the conservative for this state. But I can tell you unequivocally that Mike Espy has no business being anywhere near the United States Senate. We unite now under Trump’s umbrella. We unite now to fight for his party, and we have to win this battle for the state of our country.”

That lukewarm endorsement of Hyde-Smith was enough to assure the senator and GOP leaders that McDaniel wasn’t going to try to undermine Hyde-Smith’s campaign.

But it is unclear if McDaniel has been working the phones and contacting supporters to rally to the polls for Hyde-Smith, despite several reported overtures from her campaign to help out in the final days.

President Trump – who reluctantly endorsed Hyde-Smith over McDaniel and whom McDaniel said he “adores” – even reportedly contacted him over the weekend about the possibility of attending the president’s two rallies for Hyde-Smith on Monday.

McDaniel, who did not make an appearance at either of Trump’s rallies, did not respond to Fox News’ request for comment.

With little chance of many McDaniel supporters jumping over to vote for Espy, experts say they have to decide whether voting for Hyde-Smith or staying home is going to give them a lawmaker who will, at least partially, represent their interests.

“They have to answer the most basic question,” Duffy said. “Do they want a Republican in the Senate who they don’t agree with all the time, or a Democrat who they will never agree with?”