
On March 24, 2026, President Pro Tempore Chuck Grassley (R-Iowa) swore Sen. Alan Armstrong (R-Okla.) into the U.S. Senate, filling the vacancy in Oklahoma’s class II seat. The vacancy began on March 23, 2026, after Markwayne Mullin (R) resigned from the U.S. Senate to serve as the U.S. secretary of homeland security in President Donald…