Maurice Rosenblatt, a lobbyist and powerful behind-the-scenes Washington presence who helped engineer the downfall of Sen. Joseph McCarthy in the 1950s, died Aug. 11 at the Hebrew Home of Greater Washington in Rockville, three days after his 90th birthday. He had dementia.

For more than a half-century, Mr. Rosenblatt was at the center of many liberal-leaning political efforts. His townhouse, two blocks from the Cannon House Office Building, was a gathering place for the Washington intelligentsia for decades.

"The thing I remember vividly was that his house on New Jersey Avenue was a place where all kinds of people would meet and strategize on all sorts of issues and political campaigns," said Jack Blum, a lawyer. "Some of the people who came and went were a 'who's who' of the House and Senate."

In 1948, Mr. Rosenblatt was a principal founder of the National Committee for an Effective Congress, a political action committee designed to increase liberal representation -- among Republicans and Democrats -- in Congress. In 1959, he helped organize the Democratic Study Group to counter the influence of conservatives and southern Democrats in Congress. He rallied support on Capitol Hill for the new state of Israel in the late 1940s, lobbied for environmental reform and tried to build coalitions across party lines.

But Mr. Rosenblatt's crowning achievement was his daring backroom maneuvering to challenge McCarthy, the communist-baiting Republican senator from Wisconsin. In the early 1950s, with little or no evidence, McCarthy accused federal agencies, the military and Hollywood of harboring communist sympathizers. As McCarthy's bullying and demagoguery grew more brazen, ruining careers in the process, several lawmakers approached Mr. Rosenblatt for advice.

Among other things, he arranged an audience with Pope Pius XII to circumvent any potential backlash from Catholics. (McCarthy was Catholic.)

"I went to Rome with the support of six Catholic Congressmen and several bishops," he told the National Journal in 1987. "And all [the pope] wanted to talk about was the Brooklyn Dodgers."

In 1953, Mr. Rosenblatt formed the McCarthy Clearing House, which provided information to legislators and the media. He spoke to sympathetic members of Congress, wrote speeches and provided material to Sen. Ralph E. Flanders (R-Vt.), who introduced resolutions to remove McCarthy from his committee chairmanships and censure him for abuse of power. The resolutions passed in late 1954, and less than three years later the disgraced senator died.

"I realized I had a leadership function -- that I could rally others and motivate them," Mr. Rosenblatt said, according to a book in preparation by writer Shelby Scates. "I didn't see myself as just another Capitol Hill operator. . . . Joe was a mean guy -- a villain in the democratic process."

Mr. Rosenblatt was born Aug. 8, 1915, in New York and spent much of his youth in Austria, where his father worked as an economist helping to rebuild the country's economy.

"I had the advantage of gifted parents, a European immersion, an accelerated childhood and audacious self-confidence," he said.

He graduated in 1936 from the University of Wisconsin, where he had edited the college newspaper, and then moved to New York.

He worked for a trade union and in 1939 founded the Coordinating Committee for Democratic Action, which opposed an incipient pro-Nazi movement in the United States. He also organized demonstrations against Charles Coughlin, a right-wing priest whose vitriolic sloganeering was broadcast nationwide on radio.

At the same time, Mr. Rosenblatt disapproved of leftists' growing infatuation with communism.

After serving as a criminal investigator with the Army during World War II, he settled in Washington in 1946. From 1956 to 1993, he ran his lobbying firm, National Counsel Associates, from his Capitol Hill home. Over the years, his clients included the United Mine Workers, the National Association of Railroad Passengers, CBS executive Frank Stanton and various environmental groups.

A small, gregarious man of considerable sophistication and wit, Mr. Rosenblatt counted Sens. Eugene McCarthy (D-Minn.) and Gaylord Nelson (D-Wis.) among his closest friends and regaled people with inside tales of political intrigue.

"He was one of the great conversationalists of modern times," said Woody Price, who worked with Mr. Rosenblatt for several years. "He was wonderfully articulate. He was charming without being superficial. He was a very thoughtful person, and I think that gave him credibility."

In 1976, Mr. Rosenblatt and Curtis B. Gans founded the nonpartisan Committee for the Study of the American Electorate.

"He was an original," Gans recalled. "There was no orthodoxy that he adhered to."

Mr. Rosenblatt was married for about a month in the 1950s to model Laura Barone. After that, "he was never at a loss for female companionship," Price said, but remained resolutely single. He had no other survivors.

Mr. Rosenblatt wrote occasional articles for newspapers, including The Washington Post, and often claimed to be at work on his memoirs, which were never completed, if they exist at all.

Maurice Rosenblatt rallied others to defy Sen. Joseph McCarthy, leading to the politician's downfall.