The squat frame, gesticulating arms and firebrand rhetoric of Paul Wellstone have been a familiar presence at rallies against Minneapolis defense contractors, for farmers facing mortgage foreclosures and for striking workers at the Hormel meatpacking company.

At Carleton College, where he teaches political science, he has for years vexed administrators and attracted a worshiping cadre of left-leaning students with his stark, class-conscious speeches and confrontational style.

By any conventional measure, Wellstone, a native of Arlington, is an unlikely character to join one of the most staid clubs in America, the United States Senate. But in January, he will do just that: Prof. Wellstone will become Sen. Wellstone.

Wellstone's victory in Minnesota over incumbent Sen. Rudy Boschwitz (R) counts as one of the most jarring upsets in that state's history, elevating a little-known agitator from the fringe of liberal protest politics to a national platform.

Wellstone, who bought his first three suits this year for $100 each, was the only Senate challenger to win Tuesday.

His 52 to 48 percent victory capped a low-budget campaign during which he aired offbeat commercials making fun of his measly campaign coffers, traveled the countryside in an aging bus that kept breaking down, and advocated a frankly liberal agenda of universal national health care, military spending cuts and tax increases for the rich.

Wellstone, 46, state co-chairman of Jesse L. Jackson's 1988 presidential campaign, has been a leading grass-roots organizer in Minnesota's protest community for years. He was arrested during a farmers' protest in 1983, though the charges were ultimately dropped.

Two years ago, he and a crowd of students at the Northfield, Minn. school blocked a doorway to a board of trustees meeting to protest the college's South Africa investment policy, drawing a rebuke from Carleton President Stephen Lewis, who said the door-blockers were inhibiting free discussion.

"His stock in trade has been protest," said political scientist Steven Schier, a Carleton colleague. "He considers himself an organizer and molds himself along the model of Saul Alinsky," the late legendary Chicago grass-roots organizer.

It seems clear that many Minnesotans weren't embracing all of Wellstone's ideology in casting their votes for the Democrat. Instead, observers said, Boschwitz was caught by anti-incumbent sentiment and ill will against Republicans after the GOP's first gubernatorial nominee left the ticket in a sex scandal.

Wellstone polling director Diane Feldman said the candidate's maverick style positioned him to exploit Boschwitz's weakness. "Paul spoke to this feeling because he presented such a stark contrast," Feldman said. "He was a real outsider. . . . And it became an election about Washington and economics and change."

Wellstone's style may likewise be a change for the Senate, even with his new suits and the slightly softened tone he adopted for the campaign. "He liked to roll up his sleeves and get a gleam in his eye and rail about the evils of Reagan's America," recalled Tom Freedman, a former congressional staffer who took classes from Wellstone.

Wellstone said yesterday that he wanted to be "very outspoken" but that he has no plans to become a "gadfly -- a hit here, a hit there."

Liberal action groups in Washington also are awaiting Wellstone with anticipation. Marcus Raskin, head of the Institute for Policy Studies, said Wellstone could quickly become a leading figure on the American left.