Six years ago, Democrat Paul D. Wellstone, a frizzy-haired, hyperkinetic college professor, toppled Sen. Rudy Boschwitz (R-Minn.) with a shrewd, shoestring advertising campaign that tweaked an overly confident Boschwitz for losing touch with his state.

As he traversed Minnesota in a battered green school bus, Wellstone and his youthful followers asked, "Where's Rudy?" The electorate responded by ousting the stunned and humiliated Boschwitz.

Now, in an epic rematch, Boschwitz, a 65-year-old millionaire businessman, is playing the role of Merry Prankster -- bombarding Wellstone with stinging television and radio ads portraying him as a clown, a grungy '60s-style hippie, "Senator Welfare" and an unrepentant tax-and-spender who is "embarrassingly liberal" and out of step with voters.

And while polls show many Minnesota voters agree that Wellstone is too liberal, Boschwitz's negative ads have yet to give them a reason to vote for him.

"Rudy's problem is he hasn't developed a message beyond the bashing," said Bill Morris, a Republican market researcher in the Twin Cities area. "Boschwitz has done a reasonably decent job of explaining why voters should get rid of Wellstone, but he hasn't made the argument of why it should be Rudy."

The Minnesota contest is important as Democrats wage a long-shot effort to recapture control of the Senate or neutralize the GOP leadership by reducing its fragile 53 to 47 Senate majority. Wellstone, 52, is running a far more orthodox campaign this time, with a larger war chest than his GOP challenger. He has raised more than $6 million since taking office and had $580,656 in cash on hand as of last week. Boschwitz, meanwhile, has raised about $3.75 million for his challenge and reported $438,935 in cash on hand.

Wellstone holds a modest lead over Boschwitz in the latest polls. But national GOP strategists consider Wellstone one of the most vulnerable Democratic incumbents and have targeted him for special treatment.

"This is a very volatile race," said Gordon Hensley, a spokesman for the National Republican Senatorial Committee.

The NRSC this year has pumped more than $1.5 million of negative advertising into Minnesota media markets seeking to discredit Wellstone, one of the few Democratic lawmakers proud to call himself a liberal and the only incumbent Democrat running who opposed the welfare reform legislation signed by President Clinton. Arthur J. Finkelstein, the GOP political operative known as "the Terminator" because of his brutally effective tactics, is chief architect of Boschwitz's campaign.

Although the Republicans' jackhammer ads succeeded in driving up Wellstone's negative ratings, the strategy appears to have backfired, as Boschwitz's popularity has plummeted in a state where voters appear uncomfortable with saturation negative advertising.

A typical Boschwitz ad shows a distorted face and head of Wellstone atop a cartoon figure in a slovenly blue suit and red tie with arms that pop out with a "Liberal" sign when the announcer says: "He voted against the Balanced Budget Amendment" or "He voted against the death penalty for murderers, terrorists and drug kingpins" or "He's voted against workfare time and again." Another ad features a bearded hippie-looking actor who mockingly inducts Wellstone into the "1967 Liberal Hall of Fame."

Linda Oakes, a Minneapolis psychologist who supports Wellstone, complained that Boschwitz's ads "seem pretty obnoxious" and "I haven't heard anything about what he's going to do." Even some Republicans and independents who support or lean toward Boschwitz say they are troubled by the tone of his ads. Bill Spychalla, 49, a civil engineer from suburban Apple Valley who plans to vote for Boschwitz, said, "You sit there watching the ads on TV and wonder, who in the world comes up with this stuff?"

Last week, a survey by the Minneapolis Star Tribune showed Wellstone with a 9-point lead over Boschwitz -- 47 percent to 38 percent -- in a field crowded with minor party candidates. According to the poll, the percentage of voters rating Boschwitz unfavorably has jumped by 12 percentage points since September. Moreover, by 44 percent to 30 percent, voters who had seen the GOP's attack ads said they were more inclined to vote for Wellstone than for Boschwitz.

Wellstone also may get a last-minute boost from Clinton, whose popularity in economically prosperous Minnesota has never been higher and who leads Republican Robert J. Dole in the polls by 22 points.

Working against Wellstone is his sometimes-grating style and uncompromising stands on issues of poverty, women's rights and the environment, which have offended even some Democrats. His resistance to a bill to open up more of Northern Minnesota's Boundary Waters Canoe Area to motorized portaging infuriated politicians, resort owners and sportsmen in that traditionally Democratic stronghold and may cost Wellstone some important votes.

"Most people like Paul and find him refreshing . . . but he doesn't always say what people want to hear," said Gail Huntley of Duluth, associate director of the state's Democratic-Farmer-Labor Party.

Wellstone's great strength is a powerful grass-roots network of environmentalists, feminists, labor leaders, farmers, minorities, gays and lesbians that helped him narrowly defeat Boschwitz in 1990 and could prove decisive again this time. Female voters, who are more apt to vote than men, overwhelmingly prefer Wellstone to Boschwitz.

Liberalism may have become a dirty word in politics, but Wellstone and other liberals had a good year staring down the GOP revolutionaries on domestic spending priorities and scoring several important victories, including an increase in the minimum wage. Wellstone also instigated the successful drive for new rules to restrict gifts to members of Congress.

During a campaign stop at a senior citizen high-rise last week, Wellstone struck the pose of the happy warrior, declaring: "I am a Hubert Humphrey senator. I go to the floor of the Senate and I fight for children, I fight for senior citizens, I fight for health care. I'm a Minnesota senator!"

The rangy, graying Boschwitz, who made a fortune by founding a chain of plywood and home remodeling stores, once was more moderate in his politics but now preaches a conservative, up-by-the-bootstraps philosophy emphasizing self-help over welfare, a balanced budget, government deregulation and big tax breaks to spur the economy.

"He's mired in poverty and despair," Boschwitz said of Wellstone's preoccupation with the plight of the poor. "I don't say there's none of that in this country. . . . I believe the government has a role in all of that. I believe in compassion, but what ought to be recognized is achievement."

Boschwitz says he is waging a comeback to provide Minnesotans with a more "independent" and mainstream conservative voice, but his detractors insist his chief goal is seeking revenge for his humiliating loss.

After largely ignoring the attack ads for months, Wellstone has begun to fight back. He is running stinging ads of his own, accusing Boschwitz of being a tool of tobacco and other industries (calling him "the senator from Marlboro Country") and dredging up old votes to show Boschwitz opposed raising the minimum wage and providing medical and family leave.

After Boschwitz was quoted as saying that a bipartisan commission should look for ways to slow the growth of Medicare and other entitlements beyond what Republicans had proposed, Wellstone snapped during a debate last week, "You've got to be the only man in America who thinks that Speaker {Newt} Gingrich didn't go far enough in cutting Medicare." CAPTION: Battle for the Senate

Current breakdown (34 seats at stake)

47 Dem., 53 Rep.

Of the 53 Republicans:

13 face reelection races

Six are leaving the Senate: Colo., Kan. (2), Maine, Ore., Wyo.

Of the 47 Democrats:

Seven face reelection races

Eight are leaving the Senate: Ala., Ark., Ga.,Ill., La., Neb., N.J., R.I.