A morbid conversation has been playing out in public for the better part of a year: What happens if John McCain’s deteriorating health forces him to leave the U.S. Senate? Given the Arizona Republican’s advanced age, the aggressive form of brain cancer he has been diagnosed with, and the reality that he is currently only about 15 months into his latest six-year term, that question could sadly but reasonably be restated as when, not if. Increasingly, the GOP has been willing to acknowledge that.

The Washington Post reports this week that the uncertainty around McCain, who has been absent from the Senate since December, has “set off a flurry of hushed conversations and concerns” inside the Republican Party, particularly about whom Arizona Gov. Doug Ducey should appoint as McCain’s successor if the need arises. “The problem,” as one unnamed Republican put it, is that there is no “logical” or “obvious” choice among the party’s rank-and-file. Ducey is up for re-election himself, and if he is looking only for an interim replacement, two relatively safe names jump out: McCain’s wife, Cindy, and former U.S. Sen. Jon Kyl. But neither is seen as an ideal candidate to hold the seat long-term.

The bigger question, then, is not whom Ducey would appoint to fill McCain’s seat, but instead whom voters would select if they have the chance. And that’s where things get complicated—not just in a still-hypothetical special election but also in the heavily watched regular Senate election in November.

Under the most common reading of the state’s election laws, if McCain leaves the Senate before May 30, the ensuing special election would follow the same schedule as a regular midterm one: Party primaries would be held in August, and then the race would be decided on Nov. 6. But if McCain were to leave after May 30, his appointed successor would most likely get a free ride until 2020. Given the way the current political winds are blowing, both nationally and in Arizona, the two parties’ electoral preferences are clear even if neither will say them aloud for obvious reasons: Republicans would rather defend the seat in 2020; Democrats would prefer to expand the battleground map this fall.

The 2018 special election would significantly shake up the current battle for the Senate. Democrats need to pick up just two seats in November to gain control of the upper chamber next year but, as things stand now, there are only three GOP seats that are realistically in reach: one each in Nevada, Tennessee, and Arizona. A special election to replace McCain this fall, though, would almost certainly expand that list to four.

Under this scenario, Arizona would become the third state with both of its U.S. Senate seats on the ballot in November, joining Mississippi and Minnesota. But unlike in those states, where a non-appointed incumbent is the clear favorite for re-election in one of the two contests, both of Arizona’s two Senate races would be relatively wide open.

At the risk of going too far down this particular rabbit hole, a Senate special election running parallel to the regularly scheduled one in Arizona has the potential to get particularly messy. Consider the current crop of Republicans running to replace retiring GOP Sen. Jeff Flake, which includes establishment favorite Rep. Martha McSally and two hard-liners in former state Sen. Kelli Ward and disgraced former Maricopa County Sheriff Joe Arpaio. Presented with the opportunity, Ward, Arpaio, or even McSally could decide to jump from their current primary into the special one before the filing deadline in hopes of finding an easier path to the nomination. We’ve seen this play out on a small scale already this year in Mississippi, where GOP gadfly Chris McDaniel dropped his primary challenge to Sen. Roger Wicker to instead run for the seat formerly held by Thad Cochran, who recently retired due to his own deteriorating health.

The motivation for Ward or Arpaio would be clear—the chance to avoid the GOP favorite and a fellow firebrand—and the one who remained behind would benefit too, since he or she would have a better chance at unifying opposition to McSally among Tea Party types. Meanwhile, even McSally herself could decide to switch races if Ward and Arpaio split up, since that would give her the chance to match up with whichever one she thinks poses less of a challenge. Or both Ward and Arpaio could jump to the special election, leaving McSally with a clear path to one GOP nomination but greatly increasing the chances the other is claimed by an insurgent. And all those possibilities don’t even include X-factors like whether Republicans can find an interim senator interested in running for re-election, or whether Democratic Rep. Kyrsten Sinema decides it’s worth switching races in hopes of avoiding McSally in a general election.

It’s far too early to predict whether any of that will happen, of course. But given how much is already at stake this fall, the candidates and their parties have good reason to prepare for the possibility.