Kathryn Sullivan has helped U.S. Navy sailors navigate —literally —through rough seas as an oceanographer in the Navy Reserves. As a NASA astronaut, she’s walked in space, the first American woman to step outside a spacecraft, 140 miles above Earth.

These challenges are a world apart from what she’s now confronting in Washington as she leads an agency of federal scientists in the crosshairs of a powerful member of Congress.

But allies of the embattled chief of the National Oceanic and Atmospheric Administration say Sullivan’s experience as a working scientist exploring the upper atmosphere and the bottom of the sea and a Navy reserve officer for 18 years has prepared her well for the contact sport of Capitol Hill politics. They imprinted a military ethic of looking out for comrades and troops and a scientific one of sticking by the results of your research and never tailoring  them to others’ political whims.

“As I learned in the Marine Corps and Kathy learned in the reserves, we have core values,” said NASA Administrator Charles F. Bolden, Jr., who flew with her on two space missions, including one that deployed the Hubble Space Telescope.

“They’re not challenging a bureaucrat or a traditional political appointee,” Bolden said of Republicans in Congress. “They’re challenging a scientist. They just picked the wrong person.”

Just shy of two years into the job, Sullivan, 64, has been drawn into a lingering and passionate skepticism among some congressional Republicans of the mainstream scientific consensus that man-made pollution is behind the planet’s recent warming.

Rep. Lamar Smith (R-Tex.), chairman of the House Committee on Science, Space, and Technology, is investigating a decisive study published by NOAA scientists in June that debunked the idea of a global warming slowdown or “pause.”

Smith, a prominent climate-change skeptic, is on a campaign to discredit the scientists, whose research he claims was “rushed to publication” to advance what he calls President Obama’s “extreme” climate change agenda.

The chairman said in an email that he is “encouraged” that Sullivan has recently expressed to him “a shared commitment to ensuring that political pressure from the Obama administration is not used to manipulate the work of NOAA scientists.”

But in recent weeks he has assailed her for refusing to cooperate with an investigation he says will show that NOAA scientists “altered” historical climate data.

“I hope…we can soon expect a better response to Congressional oversight and the Committee’s lawfully-issued subpoena for documents and communications.,” Smith wrote. “I also hope that the Administrator will prove the exception to this administration’s rule when it comes to transparency.”

Prompted by the climate study, which was published in the peer-reviewed journal ScienceSmith issued Sullivan a subpoena in October demanding internal emails and records from her staff, including the scientists who led the research, which undercut a popular argument used by global warming skeptics.

Sullivan said no.

Smith then threatened her with contempt of Congress. He wrote twice to Commerce Secretary Penny Pritzker, who leads NOAA’s parent agency, threatening her with a subpoena if she did not force Sullivan’s hand.

Sullivan said no.

A crucial deadline comes Tuesday, when Sullivan must respond to the chairman’s latest demand to turn over emails of dozens of civil servants and political appointees on her staff who may have discussed the study, but not the scientists’ communications, which he has taken off the table for now.

Sullivan declined to be interviewed for this article because of the sensitivity of the dispute with Smith, her aides said.

While friends say she has not widely discussed her standoff with Smith outside the agency, they are confident she is in active discussions with the White House. “She’s probably wondering, ‘How far can I go with this?'” Bolden said.

Scientists have stuck by her in droves, with thousands writing her letters of support and the country’s most prestigious scientific societies writing a letter to Smith warning that his campaign is setting up a “practice of inquests” that will have a “chilling effect.”

Leading the science-based agency — whose reach extends to U.S. oceans, fisheries and the National Weather Service — is a sixth act for Sullivan, an avid aviator who owns a plane she flies when she can.

After a total of 532 hours in space, she left NASA in 1993 to become NOAA’s chief scientist. More recently, she held the No. 2 spot there before the Senate confirmed her as administrator in 2013.

In between NOAA stints, she spent 10 years as president and CEO of the Center of Science and Industry (COSI) in Columbus, Ohio, and another five as the first director of the Battelle Center for Mathematics and Science Education Policy at Ohio State University.

Colleagues credit her with righting NOAA’s over-budget, lumbering weather satellite program and smoothing a bureaucratic bottleneck that had brought daily dysfunction to the operations of the National Weather Service. Time magazine named her one of the 100 most influential people of 2014.

Beyond her competence as a manager, though, friends say Sullivan identifies first as a scientist.

Sen. Bill Nelson (D-Fla.), who sits on the Senate committee that oversees NOAA, said Sullivan “has enough confidence in her people and herself to know that when there is something that’s a matter of [scientific] integrity, you stand on integrity.”

John Logsdon, founder and former director of the Space Policy Institute at George Washington University and a friend of Sullivan’s for 30 years, said she “knew what she was getting herself into when she took the job.”

But he said, “I don’t know if anyone in the scientific community would have predicted the lengths that Smith would go to” to investigate NOAA scientists. Sullivan sometimes wears a small medal around her neck with an inscription that reads, “Scientist, Astronaut, Explorer.”

“Scientist comes first,” Logsdon said.

This was on display when Sullivan was second in command at NOAA in Obama’s first term, and agencies across the federal government had to write integrity policies for their scientists, to prevent political interference.

Sullivan was deeply engaged in developing and implementing NOAA’s policy , now viewed as the gold standard in government. Among its strengths, say colleagues, are a provision allowing scientists to speak openly and directly about their research at conferences and panel discussions and to journalists. This is not the case for all agencies.

“She’s defending the policy, and I think that’s appropriate,” said Jane Lubchenco, an environmental scientist and marine ecologist who was Sullivan’s predecessor as NOAA chief. “We share a belief that science should not be partisan.”