It might be uncomfortable to talk about, but Sen. John McCain (R-AZ) has been fighting brain cancer since July. The 81-year-old has not returned to the US Capitol since December.

So the speculation is already underway about what would happen if McCain stepped down — or, worse, could no longer serve. The Washington Post reported Tuesday that a list of possible successors is circulating in the whispering Republican class, headlined by McCain’s wife, Cindy McCain, and former US Sen. Jon Kyl.

If McCain leaves office before May 30, the Post indicated, his Senate seat will be on the ballot in Arizona in November 2018, as will that of his retiring junior colleague, Republican Sen. Jeff Flake. After May 30, the new senator — who would be appointed by Republican Gov. Doug Ducey — would serve through 2020.

Facing the headwinds of an unpopular agenda and an unpopular president in a rapidly diversifying state, Senate Republicans — especially after a stunning Democratic win in deep-red Alabama in December — are confronting the possibility that despite a favorable Senate map in 2018, they could lose control of the Senate. They currently hold a slim 51-49 majority.

If two Arizona seats were on the ballot, the path to a big Democratic wave would run through the Grand Canyon State: a pair of Senate seats for the taking, in a state that has been trending blue and where Hillary Clinton lost to Donald Trump by less than 4 percentage points in 2016.

We could have two Senate races in one of America’s new battleground states

Nobody wants to be caught publicly speculating about a legendary senator’s possible retirement or death, but as the Post report indicates, these are conversations happening behind closed doors in Arizona and in Washington. Given McCain’s age and the diagnosis, it’s simply a reality that he may not be able to serve for much longer. He left Washington in mid-December to receive continuing cancer treatment in his home state.

The most notable thing about the speculation thus far is the lack of consensus. From the Post:

In public, influential Republicans have been reluctant to speculate about McCain’s future in the context of electoral politics out of respect to the Senate titan, who is beloved by many in the party. But privately, they have engaged in talks about who might replace him or run for his seat. From those conversations, which have occurred among strategists, officials and donors in Arizona and Washington, a long list of names has emerged of possible interim or long-term successors, including McCain’s wife, Cindy, and former senator Jon Kyl. Interviews with nearly a dozen Republicans this week, most of whom spoke on the condition of anonymity to discuss a sensitive topic, revealed a sense of nervousness over the lack of a clear road map. “The problem,” as one prominent Arizona Republican said, is there is no “logical” or “obvious” successor.

Cindy, who has been married to McCain since 1980, has never held political office, but she has a long history in the national spotlight after her husband’s two presidential campaigns and has a record of international humanitarian work. She was rumored last year to be under consideration for a role at the State Department. Appointing a politically engaged widow to replace their spouse is relatively common — there’s even a political science term for it: “widow’s succession,” as the Washington Post’s Christopher Ingraham wrote a couple of years ago.

Kyl represented Arizona in the Senate for 18 years, eventually rising to the No. 2 Republican post. He stepped down in 2013 and has since become a lobbyist. He had a reputation while in the Senate as one of the most conservative lawmakers in the chamber, though his party has continually moved to the right since he was first elected.

Ducey, the Arizona governor, has refused to make any public comments about whom he might appoint to replace McCain, and the Post reported that he has dismissed the idea that he might appoint himself. Former Arizona Rep. John Shadegg and Ducey’s chief of staff Kirk Adams are some other names that have surfaced during this somewhat distasteful parlor game.

According to the Post, citing Arizona election officials, if McCain leaves his seat before May 30, there would be a special election this year to replace him. The primary would be held in August and a general election in November.

And the reality is that Arizona would give Democrats some of their best chances to pick up seats in their unlikely bid to retake the Senate. Flake is retiring, with a contentious GOP primary taking shape to replace him. Establishment favorite Martha McSally is facing conservative firebrand and Breitbart favorite Kelli Ward, while anti-immigration hawk Sheriff Joe Arpaio, the recipient of a Trump pardon, has also thrown his hat into the ring. Democrats also have a strong recruit in Rep. Kyrsten Sinema.

Arizona is already a crucial battleground for the 2018 midterms — a second Senate election might make it the most important state in the union. As Vox’s Matt Yglesias explained:

Republicans currently have 51 Senate seats, with Dean Heller’s race in Nevada almost certainly Democrats’ best chance to pick one up and the Flake seat in Arizona coming second. The race could, thus, very literally be the pivot point on which control of the Senate (and thus Trump’s ability to continue stocking the judiciary with Republicans) hinges — though, of course, to pull it off, Democrats need to defend a lot of incumbent Democrats in red states. More broadly, Arizona is at the center of the shifting sands of American politics. In the face of a large and growing Latino population, the state’s Republican Party appears to be shifting away from the Flake/McCain tradition of immigrant-friendly Republicanism and toward the Arpaio/Trump brand of white grievance politics.

It’s all speculation for now, and of course, no one is hoping McCain will have to step down or anything else. But the terrain of the 2018 midterms could soon change very quickly.