Sen. Paul D. Wellstone (D-Minn.), who died yesterday in a plane crash in the middle of a tough reelection campaign, was a leading champion of liberal causes in the "happy warrior" tradition of Minnesota Democratic politics.

In keeping with his maverick politics, the 58-year-old former political science professor and community organizer was a lonely dissenter in one of the last votes he cast before Congress went home to campaign last week. He was the only senator facing a tough reelection fight to vote against empowering President Bush to use military force against Iraq. To have done otherwise, he said, would have violated the principles that guided his career.

In his 12 years in the Senate, Wellstone, often described as one of Congress's last unabashed liberals, rejected the notion that government had grown too big. He stood as a rarely wavering advocate of its use to help ordinary people, especially the poor.

"Paul Wellstone was the soul of the Senate," Senate Majority Leader Thomas A. Daschle (D-S.D.) said. "He was one of the most noble and courageous men I have ever known. The nation has lost a fearless public servant and tireless advocate for justice."

"He was always willing to stand up for the little guy, even if it meant taking on the political goliaths," Sen. John McCain (R-Ariz.) said.

Even in his own party, Wellstone was often in the minority, sometimes a lone dissenter. He made long speeches -- too long in the view of some colleagues. But his speeches, frequently delivered after most senators had gone home, nearly always conveyed a personal passion and sense of commitment that stood out from the scripted rhetoric so common in Congress.

His causes were legion: universal health care, more federal spending for education, safeguards for human and civil rights, ethics in government, worker protections and better mental health care. He cast one of the few Senate votes against the 1996 welfare reform law, which trimmed benefits, and voted in 1991 against authorizing the Persian Gulf War. In one of his last fights, he held out against bankruptcy law changes that were widely supported in Congress, arguing they would benefit banks and credit card companies at the expense of financially strapped consumers.

Wellstone was the antithesis of a grim-faced ideologue. Quick to laugh and joke, he took his causes seriously but not himself, often telling stories at his own expense and kidding his foes .

He worked extensively across the Senate's political and ideological divide to pass bills on an array of issues, including a ban on gifts to lawmakers, domestic violence legislation, insurance coverage for mental illness and agricultural issues. He was widely liked and admired for his principled positions, even by his political foes.

"For the entire Senate, this is a death in the family," Minority Leader Trent Lott (R-Miss.) said. Former Senate Republican leader Robert J. Dole described Wellstone in a television interview as "a good, decent guy with different views from mine."

"You couldn't find a senator who gave more heart to his causes," said Sen. Pete V. Domenici (R-N.M.), who worked with Wellstone on mental health legislation and who, like many of his colleagues, choked up as he talked about Wellstone.

Wellstone had back pain and a slight limp from an old wrestling injury. Earlier this year, he announced he had a mild case of multiple sclerosis, which appeared to slow him down only a little.

He was a prairie populist in the tradition of former vice president Hubert H. Humphrey (D-Minn.), a sketch of whom hangs prominently in his Senate office. But he also was a product of the turbulent 1960s -- one of the decade's few activist organizers to wind up in the Senate. He was, as Mother Jones magazine noted, "the first 1960s radical elected to the U.S. Senate."

Paul David Wellstone grew up as a second-generation American in a politically active and intellectually engaged household in the Northern Virginia suburbs of Washington. His father, Russian-born Leon Wexelstein, changed the family name to Wellstone at a time of rising anti-Semitism in the 1930s. The older man spoke 10 languages, worked for the federal government and exposed his son to a wide range of scholarly pursuits. His New York-born mother, Minnie, was the daughter of Ukrainian immigrants. She worked in the cafeteria of his junior high school and helped push for desegregation of Arlington schools in the early 1960s.

After public school in Arlington, Wellstone attended the University of North Carolina at Chapel Hill. He earned a PhD in political science while participating in a variety of causes that would later mark his political career, including civil rights and poverty issues.

Wiry, short and full of energy, he took up wrestling and became a champion. In 1963, at 19, he married his childhood sweetheart, Sheila Ison, from a Kentucky coal-mining family. They had three children and six grandchildren. Sheila Ison and a daughter, Marcia, also died in the crash.

After receiving his doctorate, Wellstone went to Minnesota, where he became a political science professor at Carleton College in Northfield in 1969. He was active in the liberal causes of the day and, according to the Pioneer Press in St. Paul, was almost fired for being too focused on his activist pursuits. They included leading protests in sympathy with striking Hormel meatpackers. He was arrested while picketing a bank that had foreclosed on farmers.

Wellstone ran unsuccessfully for state auditor and managed Jesse Jackson's 1988 presidential campaign in Minnesota. In keeping with his audacious style, when he ran for office again he didn't bother with a mid-level post, but aimed high: a 1990 challenge to Sen. Rudy Boschwitz (R-Minn.). He dashed around the state in an old, green school bus (which he recycled for his campaign this year) and ran quirky ads that mocked Boschwitz's fat campaign coffers and refusal to debate. He speed-talked through his spiel, saying, "I don't have $6 million, so I have to talk fast."

Political experts scoffed but voters loved it. Wellstone pulled off the year's only upset, winning by 2 percentage points. Six years later, Wellstone and Boschwitz staged a rematch, and Wellstone won again, this time by a comfortable margin.

He made a few early mistakes, some of which would haunt him. After he won his first Senate race, he promised to serve only two terms, figuring it sounded right at the time, he later said. But he loved the Senate and didn't want to leave without accomplishing more goals, especially at a time of national crisis, he said in explaining why he broke the pledge to seek a third term in 2002.

He also infuriated veterans groups when he went to the Vietnam Veterans Memorial to oppose the Gulf War. He apologized and went on to champion veterans' causes.

In 1997, Wellstone organized a poverty tour reminiscent of a trip taken by Robert F. Kennedy 30 years earlier, and the next year he signaled his intention to run for the Democratic presidential nomination, saying that he represented "the Democratic wing of the Democratic Party."

But in early 1999, he said his back couldn't withstand the rigors of a presidential campaign and dropped out. Once again breaking with the pack in Congress, he backed former senator Bill Bradley's bid to wrest the nomination from Vice President Al Gore.

Late yesterday, Wellstone's campaign sent out an e-mail with the text of an ad he was about to run in his reelection campaign against Republican Norm Coleman. The e-mail said, "This is the way Paul wanted to end the campaign."

In the ad, Wellstone said: "I don't represent the big oil companies. I don't represent the big pharmaceutical companies. I don't represent the Enrons of this world. But you know what? They already have great representation in Washington. It's the rest of the people that need it. I represent the people of Minnesota."

Wellstone is survived by two sons, David and Mark, and six grandchildren.

Sen. Paul D. Wellstone (D-Minn.), left, talks with Richard Holbrooke, former U.S. envoy, at a Senate Foreign Relations Committee hearing on the use of military force against Iraq. Sen. Paul D. Wellstone's wife, Sheila, shown with him in New York in 2000, and daughter, Marcia, also died in the plane crash. Then-Senate candidate Paul Wellstone and his wife, Sheila, walk to their campaign bus after voting in 1990 in Northfield, Minn. Wellstone joins protesters outside the White House in a demonstration against the Persian Gulf War in 1991.