Representative Jim Bridenstine, Republican of Oklahoma, will be nominated by President Trump to serve as NASA’s next administrator, the White House said on Friday night.

Mr. Bridenstine, a strong advocate for drawing private companies like SpaceX and Blue Origin more deeply into NASA’s exploration of space, had been rumored to be the leading candidate for the job, but months passed without an announcement. If confirmed by the Senate, Mr. Bridenstine, 42, would be the first elected official to hold that job.

The previous administrator, Charles F. Bolden Jr., stepped down on Jan. 20, the first day of Mr. Trump’s presidency. Since then, Robert M. Lightfoot Jr., the associate administrator, has served as acting administrator. Mr. Lightfoot’s 225 days as of Saturday have already set a record for the longest time NASA has been without a permanent leader.

“I am pleased to have Representative Bridenstine nominated to lead our team,” Mr. Lightfoot said in a statement. “Of course, the nomination must go through the Senate confirmation process, but I look forward to ensuring a smooth transition and sharing the great work the NASA team is doing.”