A House committee staff director who quarterbacked Rep. Devin Nunes‘ efforts to reveal the story behind the Democratic Party-funded, anti-Trump dossier has died.

Damon Nelson, 46, who oversaw a staff of 20 for the House Permanent Select Committee on Intelligence, died Nov. 10 after a short illness, the California Republican congressman reported.

Mr. Nelson led the Nunes team during one of the committee’s most trying and public challenges amid constant battles with committee Democrats and the Trump Justice Department.

First, the committee tackled the probe into whether the Trump campaign colluded with Russian election interference. After assessing 73 witnesses and more than 300,000 pages of documents, the committee’s Republican majority concluded it had not, over the protest of Rep. Adam Schiff, California Democrat, the incoming chairman.

In mid-2017, Mr. Nunes began a separate probe into the Democratic Party-financed dossier written by British ex-spy Christopher Steele. He hurled a barrage of collusion charges against Trump people, none of which today has been confirmed publicly. Mr. Nunes used subpoenas to discover that Democrats paid for the dossier via opposition research firm Fusion GPS and that the FBI had embraced it.

Along the way, Mr. Nelson carried out Mr. Nunes‘ plan, as the liberal media heaped criticism on his boss.

“Damon was staff director for the entire duration of the Russia probe, so he led us through all our battles to get the DOJ, FBI, and Fusion GPS to turn over documents and information to the committee that they didn’t want to disclose,” said Jack Langer, Mr. Nunes‘ spokesman. “They were tough fights, but in the end we were successful, and Damon led the team every step of the way.”

Tributes arrived from both sides of the aisle.

Mr. Nunes issued a statement expressing his “deep sorrow” over the death of a trusted aide who happened to also be a classmate at Tulare (California) Union High School. Mr. Nunes noted Mr. Nelson’s long record of public service on Capitol Hill following a stint in the Air Force as a refueling specialist.

“I worked continuously with Damon for my entire congressional career,” the congressman said. “He was hired as my legislative director when I first joined Congress in 2003, and he later became deputy chief of staff. In those positions he was a tireless advocate for improving the California water supply and established himself as one of our region’s preeminent experts on that complicated issue. In 2015 he moved to the Intelligence Committee and a year later became staff director.”

“Damon enjoyed particular loyalty and affection from staffers both in my personal office and on the Intelligence Committee,” Mr. Nunes said. “I doubt there’s a single staff member from either office whom Damon didn’t help in matters both large and small. He was a friend, mentor, and boss for dozens of colleagues who deeply loved him and were fiercely protective of him.”

Speaker of the House Paul D. Ryan, Wisconsin Republican, said, “I am shaken by the sudden loss of Damon Nelson, staff director of the House Intelligence Committee. Damon was a dedicated public servant and a truly kind and genuine person. We are praying for his wife and family at this tragic time.”

Secretary of State Mike Pompeo, a former House Intelligence panel member, tweeted: “Damon Nelson served America loyally with honesty, dedication and commitment. His passing brings me great sadness. I worked with Damon during my time in Congress — the Intelligence Committee will miss his smile and the joy he brought to our mission. Praying for Damon and his family.”

The death came four days after Republicans lost the House to Democrats, shifting Chairman Nunes to the role of ranking member.

Mr. Schiff tweeted: “Our Committee has lost a family member, and a truly decent and caring man — a husband, a friend, and a dedicated public servant — Damon Nelson. Our hearts and our prayers are with his wife Dana, his family, his colleagues and all of his friends.”

Mr. Nelson, a University of Maryland graduated, died at the Anne Arundel Medical Center in Annapolis.