Dem
47
GOP
53
If Sen. Markwayne Mullin (R-OK) continues to agree to trade in a good job for life in the Senate for a lousy job as secretary of DHS for at most 3 years, and the Senate confirms him, that frees up his Senate seat. In most states, what happens is that the governor picks someone he or she wants to see become the new fixture in the Senate, although once in a while the governor wants a placeholder for some reason (e.g., when Dianne Feinstein died in office and Gov. Gavin Newsom, D-CA, didn't want to get involved in the succession, he appointed Laphonza Butler as a placeholder until the end of Feinstein's term).
Oklahoma law is different from that of most states. Appointed senators are not allowed to run in the next special or general election. They are only placeholders until the people of Oklahoma elect someone. Given how red Oklahoma is, the upcoming regular election for Mullins' seat in November could start a feeding frenzy with any or all of the five representatives (all Republicans) or any or all of the statewide officers (all Republicans) running in the GOP primary for a full term.
Donald Trump wants to forestall that by anointing Mullins' successor himself. His choice is Rep. Kevin Hern (R-OK), who is plenty MAGA. Prior to Trump's announcement, Rep. Stephanie Asady Bice (R-OK) was considering a run for the Senate. After the announcement she decided to stay in OK-05 (Oklahoma City). This might not be a great time for her to run statewide, though, since she is of Iranian and Pakistani descent and would be constantly asked about the famous Florence Reece song: "Which side are you on?" (V)