MT-Sen, MT-AL: On Thursday, Donald Trump said he'd nominate GOP Rep. Ryan Zinke, Montana's lone member of the House, to serve as his secretary of the interior. Assuming the appointment goes through, that would set off a special election for Zinke's House seat, and, it would seem, it would also eliminate Zinke as a potential challenger to Democratic Sen. Jon Tester in 2018. Zinke would be one of the GOP's strongest Senate options, so it's no surprise to see that at least one unnamed Republican operative is trying to keep his name alive for the race, but it would be pretty difficult to keep this slot open while Zinke goes and spends at least a year in Washington under Trump's tutelage.
And this being Montana, there are plenty of other Republicans who could run against Tester instead. Politico runs down a whole bunch: state Auditor-elect Matt Rosendale, Secretary of State Corey-elect Stapleton, Attorney General Tim Fox, state Sen. Ed Buttery, and rich guy Greg Gianforte, who just lost his bid for governor last month. The National Journal adds several more names: state House Speaker Austin Knudsen, Public Service Commissioner Brad Johnson, and state Superintendent of Public Instruction-elect Elsie Arntzen—in other words, basically everyone. And Fox, they say, issued a statement that didn't rule out a bid.
As far as the race to replace Zinke, there's a weird wrinkle. Apparently, under state law, Democratic Gov. Steve Bullock is permitted to make an interim appointment from a list of three names provided by the state GOP, but the secretary of state says that this law runs afoul of the Constitution, which requires special elections to fill House vacancies. That does indeed seem to be the case, though the statute has never been challenged in court—but it almost certainly would be if Bullock attempted to proceed. (Presumably, the Republican Party could also simply refuse to send him a list to choose from.) Bizarrely, the law only passed last year, so it's not like this is some ancient relic everyone forgot about.
Anyhow, once that issue gets cleared out of the way, quite a few Republicans will undoubtedly take a hard look at running for Zinke's seat. Anyone on the roster above could conceivably make a House bid instead of going for the Senate, especially since it would be a much easier lift. Roll Call's Simone Pathé specifically suggests Arntzen and Stapleton, who both lost the GOP primary for this seat to Zinke in 2014, and she also tosses in Rosendale.
And would Democrats try making a play here once again? The national party backed outgoing Superintendent of Public Instruction Denise Juneau earlier this year, and she wound up raising a lot of money but lost her challenge to Zinke by a wide 56-41 margin. That was somewhat better than Hillary Clinton, who lost 56-36, but Gary Johnson took a sizable 6 percent of the vote in Montana, which could explain all the difference. Just before the Zinke news broke, Juneau said she wouldn't rule out another bid, but we haven't heard from her since.