Amy Moritz Ridenour, a veteran leader in the conservative movement who testified in Congress about the illegal lobbying activities of Jack Abramoff, who had used her think tank as a conduit in his influence-peddling scheme, died March 31 at a hospital in Glen Burnie, Md. She was 57.

The cause was complications from cancer, said a brother, Christian Moritz.

At the time of her death, Mrs. Ridenour was chairman of the National Center for Public Policy Research, a Washington-based group that she founded in 1982 and that lists as its priorities a strong national defense and robust free market.

The organization, which expresses skepticism about climate change, supports private rather than federal management of environmental issues, opposes tax increases and seeks to promote African American leaders in conservatism, among other work.

Mrs. Ridenour made frequent television and radio appearances in addition to her blogging and other writing, and she often denounced what she and other conservatives regard as liberal bias in the news media.

Mrs. Ridenour testifies before the Senate Indian Affairs Committee about lobbyist Jack Abramoff in 2005. (Susan Walsh/AP)

She had been active in the conservative movement since the early 1980s, when she and Abramoff were members of the College Republicans. In her book “Gang of Five: Leaders at the Center of the Conservative Crusade” (2000), journalist Nina J. Easton described Mrs. Ridenour — then Amy Moritz — as the “most serious contender” to Abramoff in the 1981 race for the national chairmanship.

According to Easton’s account, Abramoff and a backer, future anti-tax advocate Grover Norquist, persuaded Mrs. Ridenour to withdraw from the race, offering her the position of executive director.

Abramoff won the race, and Mrs. Ridenour was “cut out of the deal before she could spell f-o-u-l,” Easton wrote. She was named deputy director; her desk was given to Ralph Reed, later head of the Christian Coalition; and Mrs. Ridenour was left with a box of supplies that was labeled “Amy’s desk.”

According to Easton, Norquist questioned Mrs. Ridenour’s loyalty to his brand of the conservative cause, although Mrs. Ridenour was, in Easton’s words, “easily as staunch a Rightist” as he was. By Easton’s account, Mrs. Ridenour did not harbor rancor after the episode, chalking up the infighting to what Mrs. Ridenour described as Norquist’s “either with him or against him” mentality. She went on with her work.

Abramoff, who emerged as a major Washington lobbyist, became a board member of Mrs. Ridenour’s think tank after its founding.

In January 2006, Abramoff pleaded guilty to charges of defrauding Indian tribes he represented, conspiracy to bribe government officials, and tax evasion. The scandal ensnared former Rep. Robert W. Ney (R-Ohio), who pleaded guilty to corruption charges in October 2006, and aides to former Rep. Tom DeLay (R-Tex.), once the House majority leader.

The investigation into Abramoff’s activities showed that he used the National Center for Public Policy Research as an intermediary for funds received from his clients and then used for Abramoff’s purposes.

According to a Washington Post report in March 2005, the Mississippi Band of Choctaw Indians and eLottery Inc., both represented by Abramoff, each sent checks of $25,000 to the think tank in May 2000. The National Center for Public Policy then covered the cost of a trip to Britain, including a golfing outing in Scotland, by DeLay, who shortly thereafter helped kill legislation that the two Abramoff clients opposed.

Mrs. Ridenour told The Post that the trip was intended as an opportunity for U.S. politicians to meet with conservatives in Britain and that the think tank would have paid for the trip with or without donations from the Abramoff clients.

Donations from the Choctaw tribe to the think tank exceeded $1 million in 2002. Emails later unearthed seemed to show, the Post reported, that Mrs. Ridenour was apparently “well aware that Abramoff viewed her organization as a convenient pass-through.”

“I might have $500K for you to run through NCPPR. Is this still something you want to do?” Abramoff asked Mrs. Ridenour in an email reviewed by The Post.

“Yes, we would love to do it,” she replied.

In 2005, Mrs. Ridenour testified before the Senate Indian Affairs Committee, asserting that she believed Abramoff had defrauded his clients as well as her organization.

“I trusted Jack,” she said. When asked if he had lied to her, she replied, “Certainly, I don’t know how I could reach any other conclusion at this point in time.”

Amy Moritz was born in Pittsburgh on Nov. 9, 1959. She began her political career after studying economics at the University of Maryland.

Her early work included serving as a regional coordinator for Republican Ronald Reagan’s successful presidential campaign in 1980. During the Cold War, her think tank promoted a conservative perspective on the U.S. fight against communism.

Mrs. Ridenour’s books included “Shattered Lives: 100 Victims of Government Health Care” (2009), written with Ryan Balis.

Survivors include her husband of 20 years, David Ridenour, who serves as president of the National Center for Public Policy Research, and their three children, Katherine Ridenour, Christopher Ridenour and Jonathan Ridenour, all of Laurel, Md.; her mother, Carol Moritz of Pittsburgh; and two brothers, Christian Moritz of Washington and Karl Moritz of Falls Church, Va.