Gaylord Nelson, 89, the three-term Democratic senator from Wisconsin who introduced mainstream America to the modern environmental movement by founding Earth Day, died of cardiopulmonary disease yesterday at his home in Kensington.

One of the leading environmentalists of the 20th century, Nelson also co-sponsored the 1964 Wilderness Act and sponsored or co-sponsored laws that protected the Appalachian Trail and banned the pesticide DDT, Agent Orange and phosphate detergents. He backed fuel efficiency standards in vehicles and strip-mining controls. He wrote the first environmental education act. He once proposed a ban on the internal combustion engine, as an amendment to the Clean Air Act.

A former Wisconsin governor and state senator, he was narrowly defeated for re-election to the U.S. Senate in the 1980 Republican landslide. For the past 24 years he worked for the Wilderness Society in Washington.

"Anyone who cares about the quality of our air, water, and land should be grateful for the life of Gaylord Nelson," William H. Meadows, president of the Wilderness Society, said in a statement.

Nelson, a compact man who has been described as having the stamina of a small bear and the public persona of a high school principal, came up with the idea for Earth Day in 1969 after visiting an oil spill off the coast of Santa Barbara, Calif. On the way home, he picked up a copy of the radical journal Ramparts and read about a "teach-in" on the Vietnam War and decided to adapt the idea for the environment.

He hired Denis Hayes, then a student at Harvard University's John F. Kennedy School of Government, to organize the event, foreseeing a friendly picnic that would appeal to Boy Scouts, mayors, college students and autoworkers. Hayes preferred something more political. Both were astounded by the turnout: 20 million Americans joined the April 22, 1970, event, cleaning creeks, recycling tin cans and learning about ecology.

"Gaylord's unique contribution is that he was the first person to see the political importance of conservation, that it could be used to mobilize people," Hayes said yesterday. "He recognized the partnership between traditional conservation issues and the new emerging urban and industrial issues. Largely forgotten is that he was the first and most important to help us build bridges between environmental concerns and organized labor."

Earth Day launched what activists called the "decade of the environment," during which 28 major pieces of legislation became law, and Nelson had a hand in many of them.

He defined environmentalism broadly. In 1970, he told college students in Denver that the environment "is rats in the ghetto. It is a hungry child in a land of affluence. It is 'public housing' that isn't worthy of the name. It is a problem whose existence is perpetuated by the expenditure of $25 billion a year on the war in Vietnam, instead of our decaying, crowded, congested, polluted urban areas that are inhuman traps for millions of people."

Nelson had been suspicious of American involvement in Vietnam since the 1964 Gulf of Tonkin Resolution, which he tried to amend to limit the military's response. Convinced by Foreign Relations Committee Chairman Sen. J. William Fulbright (D-Ark.) that the amendment wasn't necessary, Nelson withdrew it, but he was one of only three senators to vote against the $700 million appropriation that sent ground troops into Vietnam. He remained a critic of the war throughout the 1970s.

He was also a consumer advocate, urging federal tire safety standards, and he held a decade's worth of hearings on abuses in the pharmaceutical industry. He sponsored the legislation that allowed the federal government to retain control of President Richard Nixon's papers and tape recordings. In the 1990s, he spoke out about overpopulation, urging stricter immigration quotas.

Well-liked by his colleagues, he was ranked at the top of a 1971 senatorial popularity poll taken by a Rutgers University professor. Bill Christofferson, who wrote a 2004 authorized biography of Nelson, said he "never lost that small-town civility and charm, and a genuine interest in people."

Never a wealthy man, Nelson lived on his Senate salary and retired to a home that he bought in 1971 in the firmly middle-income suburb of Kensington. Even the man who took his seat in 1980, Robert Kasten, said at the time that Nelson was as close to a living legend as was likely to be found in American politics. And Wilderness Society president Meadows noted that after Nelson left office, instead of cashing in with a lucrative job as a lobbyist or lawyer, he chose to work for a nonprofit organization.

Born to a country doctor and nurse in the small town of Clear Lake, Wis., Nelson grew up immersed in the outdoors and enamored of his home state's progressive hero Robert M. "Fighting Bob" LaFollette.

"All we had was the out of doors," he told the Milwaukee Journal Sentinel in 1999. "We had no radio until I got into high school. There was no television, obviously. And once the first big snowfall came in winter, there were no automobiles because they didn't plow the road. So you made your own entertainment."

He graduated from California's San Jose State College in 1939 and from the University of Wisconsin's law school in 1942. During World War II, he served in the Army in Okinawa, Japan, where he commanded a unit of black soldiers and met his future wife, an Army nurse. Their romance was one of the stories told in Tom Brokaw's "The Greatest Generation" (1998).

After the war, Nelson returned to Wisconsin and set up a law practice in Madison. There he met Aldo Leopold, author of one of the first ecology books, "A Sand County Almanac," which made an indelible impression on him, Nelson said.

He was elected to the state senate in 1948 and after three terms won a race for governor, becoming the second Democrat elected to that seat in the 20th century.

"I had concluded that the most important issue facing us as a society was the environmental issue," he told the Milwaukee paper. "My concern was the political establishment was not interested in the environment, whereas the public was."

He committed the state to buying a million acres of parkland, wetlands and wildlife habitat, funded by a 1 cent tax on cigarette packs. The Outdoor Recreation Act Program raised $50 million and became a model for other states.

In the Wisconsin state senate, Nelson served with Melvin Laird, who would later become Nixon's secretary of defense. Laird said they would argue vociferously on the senate floor, then adjourn to dinner and drinks.

"There was no closer political friendship and love between two opposite party members in the history of Wisconsin politics than that of Gaylord and me," Laird said.

The friendship continued in Washington. Late one night at the Army and Navy Club, after arguing whether the "hotline" to Moscow was at the White House or the Pentagon, Laird summoned his driver, loaded Nelson into the back seat with him and took him over to the Pentagon command center, where service members on duty must have been stunned to see the defense secretary and the antiwar Democrat stroll in.

"I said, 'Right there is the hotline, and I'm going to have them run through an experiment with it right now' and have them call Moscow," Laird said, and Nelson finally admitted he was mistaken. He often told the story in later years, Laird said.

In 1995, President Bill Clinton awarded Nelson the Presidential Medal of Freedom, the government's highest civilian honor, for his environmental leadership. A Wisconsin state park and the University of Wisconsin's environmental studies institute were named for him, and next month, the Apostle Islands wilderness area will also bear his name.

Nelson realized the importance of his role in the environmental movement, telling the Minneapolis Star-Tribune 10 years ago, "We've seen a tremendous change in every aspect of every social and cultural group in the country during the past 25 years. It's subtle, sort of like watching lily pads grow but not being able to see them move on any particular day."

Survivors include his wife, Carrie Lee Nelson of Kensington; three children, Gaylord Nelson Jr. of Dane, Wis., Jeffrey Nelson of Kensington and Tia Nelson of Madison, Wis.; and four grandchildren.

Gaylord Nelson helped sponsor several environmental laws.

President Bill Clinton presents Gaylord Nelson the Presidential Medal of Freedom at the White House in 1995. Nelson looks over the shoreline on Devil's Island, part of Wisconsin's Apostle Islands wilderness area, which will be named for him this month.