Tim Steller

Tim Steller of the Arizona Daily Star.

Rick Wiley / Arizona Daily Star

Even before Sen. John McCain’s diagnosis of brain cancer this week, many people suspected he would not finish his term.

Yes, when he won his sixth six-year term last year, he was still sharp and bullheaded. In squashing his Republican and Democratic challengers, he made it look like he could serve till he’s 100.

But he was 80 years old on Election Day, and retiring mid-term would allow the governor to give his Republican successor a leg up on re-election. It seemed pretty possible.

Now that calculus has changed. McCain’s diagnosis with an aggressive brain cancer makes it very likely he will not finish his term.

If not, what happens? Actually, it’s pretty simple. Gov. Doug Ducey appoints a replacement, who by law must be a Republican, like McCain. The appointee then must stand for election at the next general election.

If McCain were to retire in the next year, then whoever Ducey appoints will have to run in November 2018.

Really, that system is simple. What’s more complicated is thinking through who Ducey might choose to replace McCain. This is something that’s been idly discussed since Nov. 8, but now those discussions are much more urgent.

I can’t claim to know what Ducey’s been thinking or talking about when it comes to possible new U.S. senators. But the likely candidates aren’t hard to figure out. So I’ve come up with two tiers of possible candidates and a wild card, listed in no particular order.

Top tier — mostly congressmen

Kirk Adams — Ducey’s chief of staff has a few big things going for him: He is close to the governor, he is allied with the dark-money groups that can boost campaigns, and he’s a known conservative largely acceptable to state Republicans.

Rep. Trent Franks — The Glendale Republican is perhaps the favorite Arizona congressman among evangelical Christian conservatives. Ducey would endlessly please the Center for Arizona Policy and its supporters if he picked Franks.

Reps. Paul Gosar, David Schweikert — Both are conservatives who represent relatively safe Republican districts. That means, if Ducey appointed either, it wouldn’t put their seats at risk. But Schweikert has the electoral advantage in a statewide race of being from the Phoenix area, while Gosar has lived in Flagstaff and Prescott.

Rep. Martha McSally — This possibility deserves a little deeper reflection because she is ours, after all.

Cons: McSally represents a swing district, which means it will be harder for Republicans to retain the seat if she leaves for the Senate. Also, she’s from Southern Arizona and therefore a harder sell in Maricopa County. Finally, she’s a moderate compared to the other possible candidates.

Pros: McSally is a former military aviator, like McCain. She is a woman, of course, and she has shown a strong ability to raise money. Also, she might lose anyway next year, so why not appoint her? (See above.)

Second tier — mixed bag

This tier contains some long shots, some curiosities, and some place-holders — people who might serve until the next election.

Andy Tobin — A former speaker of the state House, Tobin has played several different roles for Ducey and now serves on the Arizona Corporation Commission. The governor could count on him to keep the seat warm for the next candidate.

Robert Graham — Graham is the former chair of the state GOP and played a prominent role in the party nationally until leaving office after the presidential election. He is known by Republican activists and could be a placeholder or possibly a longer-term selection.

Jeff DeWit — The state treasurer and Ducey famously tangled over Prop. 123, which Ducey proposed and DeWit opposed. They’re not friends, so the chance Ducey would appoint him is pretty tiny. But appointing DeWit would mean removing a possible challenger to Sen. Jeff Flake in 2018, and it would put a Donald Trump loyalist in one of Arizona’s Senate seats.

Sen. Debbie Lesko — A leader of the state Senate, Lesko famously pushed for increasing the school voucher program last year and has been the state chairman for the Arizona Legislative Exchange Council, or ALEC, a group loved by Arizona conservatives and hated by Democrats.

Matt Salmon — The former congressman retired last year to spend more time in Arizona with his family. However, he could be persuaded to at least be a placeholder.

Rep. J.D. Mesnard — The speaker of the state House, Mesnard is young for the job, at 37, but that could be a strength. Ducey has shown a preference for young supply-side nerds in his hires.

Wild card

Doug Ducey — The governor can appoint himself and would not be the first governor to do so. It would allow him to make a bigger name on the national stage, which he has been preparing for. However, he is more of a chief-executive type, and being a senator wouldn’t necessarily help in a presidential run.

Contact: tsteller@tucson.com or 807-7789. On Twitter: @senyorreporter