“States have a range of laws about replacing a departed senator, but the large majority — 37 — call on the governor to pick a successor. Of those, only seven require the governor to pick someone in the same party. So there are 30 states where the governor can pick whatever new senator he or she wants.
What that adds up to, in practical terms, is that in nine states (as of Jan. 15), a Republican governor has the authority to replace either one or two Democratic senators. If a single Democratic senator in any of those states had to leave office, the Republican governor of that state could appoint a GOP replacement that would immediately give the party a 51-49 Senate majority.”