Mr. Bridenstine takes over an agency in transition. While President Obama talked of sending astronauts to Mars in a couple of decades, the Trump administration has instead focused on a nearer, quicker goal: to return to the moon. The administration has also proposed getting NASA out of the business of running the International Space Station and instead spur commercial alternatives that do not yet exist.

Critics have questioned whether the agency’s new administrator is up to the task.

Mr. Bridenstine, a former Navy pilot who is now in his third term in the House of Representatives, has become immersed in space issues. In 2016, he sponsored a bill called the American Space Renaissance Act, which proposed broad, ambitious goals for the nation’s space program, including directing NASA to devise a 20-year plan. Although it did not reach a vote, some of the ideas were incorporated into other legislation.

But Democratic senators, led by Bill Nelson of Florida, opposed Mr. Bridenstine for several reasons. For one, they said Mr. Bridenstine was too political — he would be the first elected official to serve as NASA administrator. During the confirmation hearings in November, Mr. Nelson read back Mr. Bridenstine’s disparaging remarks about other politicians, even other Republicans.

“I think what’s not right for NASA,” Mr. Nelson said during a speech on the Senate floor on Wednesday, “is an administrator who is politically divisive and who is not prepared to be the last in line to make that fateful decision on ‘go’ or ‘no go’ for launch.”

Mr. Bridenstine also has no experience running a large government bureaucracy. The expectation was that someone with that type of background would be tapped to be NASA’s deputy administrator to handle more of the day-to-day management. However, the Trump administration has yet to nominate anyone for that position.