OKLAHOMA CITY (AP) — Oklahoma’s governor on Tuesday appointed energy executive Alan Armstrong to serve in the U.S. Senate through the end of the year and finish the term of Republican Markwayne Mullin, the new homeland security secretary.

The choice by Republican Gov. Kevin Stitt, who had pledged to pick a “conservative voice” to fill Mullin’s seat, elevates to the Senate the chairman and former CEO of Williams Companies, a major pipeline operator based in Tulsa.

“He’s a strong business leader who understands the power of free markets and limited government,” Stitt said.

Mullin, who was confirmed on Monday to take over the embattled Department of Homeland Security, would have been up for reelection in November.

The last nine months of his term will now be filled by Armstrong, who under Oklahoma law must agree to not run for a full term this fall. Republican U.S. Rep Kevin Hart quickly announced his candidacy for the Senate seat and has already been endorsed by President Donald Trump.

Armstrong, who has never served in elected office, has spent his career with Williams Companies, which employs about 5,800 people and specializes in the collection, storage and transportation of natural gas. He became president and CEO in 2011 and transitioned to executive chairman last year.

He is a longtime supporter of Stitt, giving him $8,500 in total campaign contributions since 2018, the maximum amount allowed under Oklahoma law.

Stitt, who will leave office in January and is head of the National Governors Association, made the pick weeks after Trump lashed out at him following a dispute over who was allowed at the group’s annual meeting.

Former Tulsa Mayor Dewey Bartlett, Jr., described Armstrong as an “even-tempered individual” who is more likely to build consensus around a decision rather than being driven by political ideology.

Bartlett, whose father served in the U.S. Senate in the 1970s, said Armstrong’s knowledge of the energy industry and its market factors also make him an ideal selection, particularly as the Iran war rattles the global flow of oil.

“I think that certainly is a great approach, finding consensus for making a decision,” said Bartlett, the president of an oil and gas company.

In confirmation hearings last week, Mullin tried to present himself as a steady hand to the lead agency following the firing of Kristi Noem, who came under intensifying backlash over immigration enforcement and mass deportation operations carried out under her watch.

Sean Murphy, The Associated Press