Howard G. Paster, 66, an influential lobbyist for unions and big business who was credited with pushing through the landmark North American Free Trade Agreement as President Bill Clinton’s chief liaison to Congress, died Aug. 10 at Johns Hopkins Hospital in Baltimore. He had encephalitis caused by a brain tumor.

In a Washington career spanning more than four decades, Mr. Paster was a formidable presence on Capitol Hill and in the lobbying corridors of K Street. He became chief executive at Hill and Knowlton, one of Washington’s biggest public-relations and government affairs firms, and later was executive vice president of its parent company, the WPP Group.

Known since college as “Faster Paster” for his hard-driving style, he was a congressional aide before becoming the top Washington lobbyist for the United Auto Workers in the late 1970s.

During negotiations over the government’s loan guarantees to keep Chrysler from impending bankruptcy, Mr. Paster fought successfully to halve $1 billion in proposed wage cuts for autoworkers. His performance, at 35, so impressed the lobbyists at Timmons, the firm on the other side of the table, that they hired him away at the end of the talks.

At Timmons, Mr. Paster was one of the few Democrats among mostly Republican lobbyists and became the firm’s go-to man for winning votes on the left side of the aisle. He worked for clients including Morgan Stanley, Major League Baseball and the National Rifle Association, and he achieved a major victory for oil companies for his role in diminishing some environmental regulations from the 1991 Clean Air Act.

When Hill and Knowlton found itself beset by controversy in the early 1990s, Mr. Paster was hired to right the ship. The firm had lost more than half of its clients and more than half of its staff in the wake of criticism over the exorbitant fees it charged and for taking on clients few others would have, including the tobacco lobby and the scandal-plagued Bank of Credit and Commerce International.

Mr. Paster accepted the job as chairman of public affairs and general manager of its Washington office. That was in August 1992, less than three months before Clinton was elected president.

Mr. Paster wrote into his contract that he was free to leave if he was offered a job in the administration, The Washington Post reported.

He was recommended for the White House job by Rep. John D. Dingell (D-Mich.), a long-serving member of the U.S. House of Representatives whose state is a major auto manufacturer. Clinton was, in part, driven by a desire to choose a consummate Washington insider such as Mr. Paster to help smooth passage of his legislative agenda.

Mr. Paster’s stint as the president’s chief lobbyist in Congress did not start smoothly. In April, he took some blame when Senate Republicans filibustered the president’s economic stimulus plan. At the same time, he drew criticism from conservative Democrats, who said he didn’t sufficiently convey their concerns to the White House.

“It’s fair to say that the fact the stimulus bill did not pass is my responsibility,” Mr. Paster told the New York Times. “I miscalculated over what it would take to negotiate a bill. The mistake I made was misreading what was going on in the Republican caucus.”

Defenders said that the herding of Capitol Hill’s political cats was a thankless job, and putting blame on Mr. Paster was easier than faulting the president.

Mr. Paster played a major role in the passage of the NAFTA trade agreement, which eliminated trade barriers between the United States, Canada and Mexico. It was a contentious piece of legislation vilified by detractors for the consequences they believed it would have for American workers. The “giant sucking sound,” said then-presidential candidate Ross Perot, was the sound of Mexico pulling jobs from the United States.

“His style was one of incessant phone calls [and] persuasion,” said Bill Richardson, the former governor of New Mexico and a Democratic congressman who voted in favor of the trade agreement, in an interview. “But mostly, he was most effective person to person. He would charm you to death. When he said the president needs you, he’d do it with a smile . . . and made you feel that the president really did need you.”

Mr. Paster found the intensity of the White House job exhausting. The NAFTA victory gave him an opening to leave on a high note, and at the end of 1993, he returned to lobbying.

“If we’d lost NAFTA, Howard and I would have been on the top of the South Portico with our hands tied behind us, and we would have been pushed over the edge onto a bed of knives,” Roy M. Neel, another administration official who left the White House around the same time, told the New York Times. “We would have been convenient sacrificial lambs, but we were spared that by the United States Congress.”

In a statement, Clinton said Mr. Paster made a “tremendous difference” in carrying some of the administration’s most “crucial initiatives.”

Howard George Paster was born Dec. 23, 1944, in Brooklyn. He grew up in a left-leaning family in Republican Nassau County, where his mother was a Democratic precinct worker.

Mr. Paster met his future wife, Gail Kern, when they co-edited their high school newspaper.

Besides his wife, who is now director emeritus of Folger Shakespeare Library, survivors include two children, Emily Paster of River Forest, Ill., and Timothy Paster of Manhattan; two sisters, Stephanie Paster and Ann Brody Cove, both of Bethesda; and four grandchildren.

Mr. Paster edited the newspaper at Alfred University in New York and graduated in 1966 as a government major. He received a master’s degree from Columbia University’s journalism school.

He became “seduced by Washington,” his wife said, after his early jobs working for Rep. Lester Wolff (D-N.Y.) and Sen. Birch Bayh (D-Ind.).

“Howard is very candid,” Bayh once told The Post. “In the Washington environment, where people get intoxicated with their own sense of importance, Howard is one to stand up and say, ‘Hey, senator — your zipper’s down!’ Senators don’t like being told that their zipper’s down. But a senator is a fool if he doesn’t recognize and appreciate how valuable an asset it is if someone tells him that.”