Alan Armstrong was sworn in Tuesday to temporarily fill the seat left vacant by Sen. Markwayne Mullin’s move to lead the Department of Homeland Security.
The Republican energy executive took the oath of office from Sen. Chuck Grassley (R-Iowa) just hours after Oklahoma Gov. Kevin Stitt announced Armstrong as his choice to succeed Mullin.
Armstrong will serve until a successor is elected in November. Rep. Kevin Hern (R-Okla.) is running and is viewed as the favorite after securing President Donald Trump’s endorsement.