“Someone in that position needs to set a strong ethical tone, from the top, about the proper use of taxpayer dollars,” Mr. Schwellenbach said in an interview.
“This is an old story, no there there,” said Sheryl Kaufman, Mr. Bridenstine’s communications director. “The accusations have been fully refuted by members of the Board of Directors of the museum both in 2012 and again in September 2017.”
Opponents also painted Mr. Bridenstine as a denier of climate change. In a 2013 speech in the House of Representatives, Mr. Bridenstine sharply criticized President Obama, saying his administration was spending too much money on the issue and said Mr. Obama should apologize.
Mr. Bridenstine has since moderated his public views, saying he supports NASA research into the causes of extreme weather. During his confirmation hearing, he agreed that human activity “absolutely” contributed to climate change, but sparred with Senator Brian Schatz, Democrat of Hawaii, over whether it was “a contributor” or the “primary cause.”
Some opponents also cite Mr. Bridenstine’s conservative social views like opposition to same-sex marriage. “I stand squarely in support of traditional marriage,” Mr. Bridenstine wrote on his Congressional website in July 2013.
In his confirmation hearing, Mr. Bridentstine tried to make a distinction between views he espoused as a politician and how he would act as the manager of a large federal agency. “I want to make sure that NASA remains, as you said, apolitical,” Mr. Bridenstine said to Mr. Nelson.
The previous administrator, Charles F. Bolden Jr., stepped down on Jan. 20, 2017, the first day of the Trump presidency. Since then, a longtime NASA official, Robert Lightfoot Jr., has been filling in. By the time Mr. Bridenstine was officially nominated, on Sept. 5, Mr. Lightfoot had already served 228 days, the longest span for an acting administrator.
Mr. Lightfoot then served for another 226 days, until Thursday.
Mr. Bridenstine’s nomination languished, because although the Republicans hold a 51-49 majority, he did not appear to have the necessary votes for confirmation. Senator John McCain, Republican of Arizona, was away for cancer treatment, while Marco Rubio, the other Florida Senator, a Republican, also expressed reservations about putting a politician at the top of NASA.
Then, last month, Mr. Lightfoot announced plans to retire at the end of April. That appears to have led Mr. Rubio to reluctantly change his mind.
Still, Mr. Bridenstine’s confirmation was accompanied by last-minute drama.
On Wednesday, a preliminary vote to limit debate on his nomination unexpectedly deadlocked at 49-49 when Mr. Flake voted against it. (Two senators, Ms. Duckworth, and Mr. McCain, were absent.) Vice President Mike Pence, who could have broken a 49-49 tie, was also out of town.
Mr. Flake then changed his vote to yes. He did not offer a detailed explanation.
Other than the confirmation hearing, Mr. Bridenstine has spent much of the last seven months keeping quiet. He largely stopped making any public statements and voting on bills to avoid conflicts of interest.
He attended the first meeting of the National Space Council meeting in October, a panel revived by the Trump administration to coordinate space issues between various federal agencies, but did not speak or participate. And during Mr. Trump’s State of the Union address in January, he brought a guest: Bill Nye “the Science Guy.”
Correction: April 19, 2018
An earlier version of this article misstated the year during which NASA’s previous administrator, Charles F. Bolden Jr., stepped down. It was in 2017, not 2016.