Gov. Kevin Stitt appoints energy executive Alan Armstrong to replace Markwayne Mullin in U.S. Senate

Appointment follows Mullin’s move to Homeland Security post

Gov. Kevin Stitt on Tuesday, March 24, 2026, appointed Tulsa energy executive Alan Armstrong to serve in the U.S. Senate, filling the vacancy created when Sen. Markwayne Mullin left the chamber to join the Trump administration as secretary of the Department of Homeland Security.

The appointment places Armstrong in the seat through the end of 2026, completing the remainder of Mullin’s term. Under Oklahoma’s current vacancy procedure, the governor selects a temporary replacement while the seat is decided in the regularly scheduled November election.

Who Alan Armstrong is

Armstrong is best known for his long tenure at Williams, the Tulsa-headquartered pipeline company. He previously served as the company’s chief executive and later as executive chairman of its board, building a national profile in the midstream natural gas sector and related energy infrastructure policy circles.

Stitt’s choice aligns with the governor’s stated intention to appoint a Republican replacement reflecting the outgoing senator’s party affiliation and the state’s voting history in recent federal elections.

What the selection means for the 2026 ballot

The appointment does not settle who will hold the seat in the next Congress. Voters will choose a senator in November 2026 to serve the next term beginning in January 2027, when the new Congress is seated.

Oklahoma’s vacancy law also includes a requirement that an appointee serve as a caretaker rather than using incumbency to seek the seat in the imminent election. The practical effect is that Armstrong is expected to serve temporarily while candidates campaign for the full term.

Early political maneuvering already underway

The vacancy has quickly triggered activity among Oklahoma Republicans. U.S. Rep. Kevin Hern has announced he is running for the Senate seat, becoming one of the first major contenders to enter the race. President Donald Trump has publicly endorsed Hern, a move that could shape the Republican primary dynamics in a state where GOP nominations often determine the general election outcome.

Other prominent Republicans have been discussed publicly as potential entrants since Mullin’s nomination for the Homeland Security job became public earlier this month, reflecting a wide-open contest to succeed him.

• Armstrong will represent Oklahoma in the Senate for the remainder of 2026.
• The seat will be on the November 2026 ballot for a full term starting in January 2027.
• Hern has declared his candidacy and has received Trump’s endorsement.

The appointment closes an immediate vacancy in Oklahoma’s Senate representation while setting up a high-stakes intraparty contest ahead of the November election.

Mullin, first elected to the Senate in 2022, was confirmed for the Homeland Security role on Monday, March 23, 2026, clearing the way for his departure from Capitol Hill and Stitt’s appointment.