Watching submarine races
Lisa Murkowski last ran in 2016, so she will be up again in 2022. Murkowski is the closest thing the Republicans have to a moderate Senator these days.Gardner and Collins are up for re-election if memory serves. I don't know about the other two.
However, in 2010 she was defeated in the primary by a Tea Partier. She won in the general election as a write in! Her most reliable sources of support are Native corporations and PACs, teachers' unions, and firefighters' unions, so she is not relying on typical Republican donors
One of her current interests is reviving Heidi Heitkamp's bill to help solve crimes against Native Americans (Heidi, a Democrat, was defeated in 2018.)
"It's disappointing that one Republican member of Congress blocked Savanna's Act from passing this year," Heitkamp said in a statement. "But fortunately, Rep. Goodlatte won't be around to block it in the new Congress. I've talked with Sen. Murkowski about Savanna's Act and I'm so proud that she will reintroduce my bill in the new year."
Shelley Moore Capito represents WV, a state which voted strongly for Trump, and yet who reelected Sen. Joe Manchin (D) last year. She is up for election in 2020. Her 2014 win was rather narrow.
WV has a high percentage of people on foodstamps (who are threatened by this shutdown).
and WV implemented the Medicaid expansion, so it is an ACA using state (6th highest percentage-check second graph)
It is an interesting state.