Murkowski is younger and under more primary pressure, whereas I think Manchin is happy to sail into the sunset in 2024 so party switching would do nothing for him.
I think if Murkowski does leave the Republican party though it will be to sit as an Independent.
Just curious, does Murkowski have to caucus with anyone? Could you have a senate where literally no party had a majority?
Senators are not required to caucus with a party. Dean Barkley during his two months in the Senate in 2002 did not caucus with Democrats nor with Republicans. So that scenario is theoretically possible.