Marilyn Nieves/Getty Images

(WASHINGTON) — Democrats and Republicans can at least agree on one thing: They have no idea what’s going to happen in next year’s Arizona Senate race.

The election is shaping up to be an unpredictable three-way contest in one of the nation’s premier battlegrounds featuring an incumbent who left her party, a polarizing conservative who remains a rock star with her base and a Democratic nominee-in-waiting who would represent a jolt to the left for the historically moderate-minded state.

“This is kind of a pick your own adventure,” said John LaBombard, a former aide to incumbent Sen. Kyrsten Sinema. “If a two-way race is impossible to predict, this is exponentially harder to predict.”

Sinema first set the stage when she switched from being a Democrat to an independent late last year. While she called that choice a “reflection of who I’ve always been,” the switch also prevented a primary fight with Arizona Rep. Ruben Gallego — and paved the way for something more unusual.

Sinema has not yet said if she plans on running for reelection. If she does jump in, she’d ensure a race with three major candidates in a state where voters unaffiliated with the two major parties outnumber both Republicans and Democrats, complicating tidy assumptions about who would emerge the winner.

State data shows just under 35% of Arizona voters are registered Republicans and 30% are registered Democrats, while 35% aren’t registered with either.

If Sinema retires, the race to succeed her could instead feature Gallego, a Marine Corps vet and former House colleague of Sinema’s who has become vocally critical of her, and Republican Kari Lake, a former TV anchor, election denier and 2022 gubernatorial candidate.

Both Gallego and Lake are more associated with their parties’ ideological flanks than the centrists who have historically won statewide.

“It’s gonna be like nothing we’ve ever seen before in Arizona,” said state GOP strategist Lorna Romero. “I think what’s going to make it nasty is obviously Ruben and Kyrsten don’t get along personally. And depending on who the Republican is, if you get a firebrand like a Kari Lake again, we’ve seen how she’s operated before, that’s going to take it to another level.”

Nineteen operatives from both parties who spoke with ABC News for this story mostly thought that Sinema would run again, pointing to her ongoing fundraising and continued involvement in high-profile legislative pushes like on immigration.

The focus on the fate of Arizona’s upcoming Senate race is hardly surprising given the outsized attention Sinema has drawn since she flipped her seat in 2018.

Sinema, who previously served in the House for three terms, quickly thrust herself into the heart of several of the chamber’s most significant and successful legislative efforts, including on infrastructure, same-sex marriage, guns and more.

Her style of legislating won her both plaudits and reproach, with supporters pointing to her scorecard and liberal detractors saying she has been overly eager to water down Democratic priorities, including raising the minimum wage, supporting prescription drug pricing reform and scrapping the Senate filibuster as a way to codify abortion protections.

“Overall, I think the state of Arizona is content with the work that Sen. Sinema done,” said Cesar Chavez, a former Democratic state lawmaker. “The issues that Sen. Sinema … has advocated for, and has been very successful in doing so, will definitely result in a positive tick in her numbers.”

“I cannot stress this enough how deeply unpopular she is,” countered Steve Slugocki, a senior adviser to Arizona Secretary of State Adrian Fontes, a former chair of the Maricopa County Democratic Party. “I traveled the state everywhere with Adrian last year. The first question was always, ‘What are we going to do about Sinema? How can we replace her?'”

Those countervailing analyses are fueling a Democratic debate over the electoral paths for Sinema and Gallego.

Sinema’s supporters point to independents’ registration advantage and say that her more moderate — even unorthodox — policy stances match a state where Democrats have had success in recent years even as the environment is far from solidly blue.

“They say actions speak louder than words, and she has reached across the aisle probably more than anybody else I know in the entire Congress to forge solutions,” said one former colleague, who asked not to be quoted by name. “She can run on a platform of ‘promises made, promises kept.’ And I think it’s a very strong message.”

Others, however, say that most independents in fact vote for one major party or the other and that while she may keep true independents and take a chunk of moderate Republicans turned off by a GOP nominee like Lake, not enough Democrats would defect to vote for a third-party candidate — aiding Gallego.

“If [Gallego] can keep his Democrats, he’s pretty solid, because if they lose 30% of Republicans on the R side, Ruben only loses 10%, well, he’s probably gonna have more than everybody else at the end of the day,” said nonpartisan pollster Mike Noble. “Independents in the middle, they’re not truly independent. One-third of them are Democrats, one-third are Republicans and one-third are actually true independents.”

All of this will play out in the larger context of the battle for the Senate, with the Democratic caucus (which includes three independents) holding a 51-49 majority but defending 23 seats in 2024, making operatives eager to avoid a spoiler candidate but without any agreement on who that would be.

“She [Sinema] has shown she knows what it takes to win in Arizona. I look at these other candidates in both major parties, including my own, and I do not see proven winners,” said LaBombard, one of her former aides. “So yeah, as a Democrat, I’m nervous because I want to keep Kari Lake out of the Senate.”

“If she’s in the race as an independent, Ruben’s already at a disadvantage as a Democrat just because of the numbers that we have,” said one former state lawmaker who is supporting Gallego. “So, it comes down to a point of how many votes is Sen. Sinema going to take? Even if it is a very small percentage, any small percentage at all could tip this.”

To be sure, Democrats aren’t the only ones wringing their hands.

Republican strategists told ABC News that a three-way race with a Democrat and an independent with a Democratic background would normally be a boon to the GOP candidate. But Lake is looming in the wings and is coming off a narrow loss to Democrat Katie Hobbs, which many observers attributed, in part, to her more hardline positions including her embrace of baseless claims of 2020 election fraud.

A source familiar with Lake’s thinking told ABC News they’re confident she’ll run for the Senate, likely launching a campaign in the fall. This person also confirmed that Lake recently met with several senators, including National Republican Senatorial Committee Chair Steve Daines of Montana, and that with her broad name recognition and support from conservative voters, she would enter the race as the overwhelming primary favorite.

Republican strategists agree with that assessment of the primary — and the attendant risks in the general.

“[T]he Sinema breakup from the Democratic Party looks good for Republicans on paper, theoretically. But I think that the big heartburn is what’s going to happen out of a Republican primary, and are we just going to do the same ridiculous mistakes over and over again, and Democrats are just going to continue to win in the state,” Romero said.

Still, virtually every person who spoke to ABC News added the caveat that their analyses could end up being off given the unpredictable nature of a potential race with the incumbent running as the third-party candidate.

“We’ve never seen anything like that in Arizona,” one Democratic strategist in the state said. “I just think trying to predict anything right now, you might as well shake a Magic 8 Ball and see what it tells you.”

Copyright © 2023, ABC Audio. All rights reserved.