Sen. David F. Durenberger, once considered almost unbeatable, woke up about a month ago to find his reelection bid in deep trouble. He hasn't recovered since.

He was trapped between economic bad times and a multimillionaire Democratic challenger who was spending his way to a national record.

Durenberger, a first-term Republican, has tried almost every trick in the book to escape. He has tried the high road and the low road; negative advertising and positive advertising. He has attacked President Reagan and praised him.

He has even enlisted a surrogate, Rep. Bill Frenzel (R-Minn.), to do his dirty work. Though claiming to be "a one-man truth squad," Frenzel has used falsehoods, innuendo and ridicule in an attempt to portray Durenberger's Democratic-Farmer-Labor opponent, Mark Dayton, as "a guy who has come from nowhere and spent his way almost to glory."

It has made Durenberger, a moderate frequently at odds with Reagan, look uncertain and confused, like a beagle who has lost a scent.

"I knew September had been a bad month, but I was genuinely surprised when I found he was within four points," the senator said here the other day.

Dayton, 35, has now spent $5.1 million of his own money -- a record for a Senate candidate -- putting together the most expensive and professional campaign this state has ever seen. According to a poll taken for Dayton, who trailed by 29 percentage points in May, he has moved within two percentage points of the incumbent. Durenberger says his polls show him leading by 9 percent, and an independent polls estimate him leading by 7.

Dayton, seeking his first elected office, decided to make a long-shot bid for the Senate three weeks after the 1980 elections, and has been campaigning ever since as "a strong new voice for Minnesota."

He is an heir to the Dayton-Hudson department store fortune; his wife, Alida, is the daughter of John D. Rockefeller III, and sister of West Virginia Gov. Jay Rockefeller. The couple met at a seminar for socially concerned children of the very wealthy. They have a combined fortune of $19 million to $30 million, according to Federal Election Commission records.

Dayton, who defeated former senator Eugene J. McCarthy in the DFL primary, has an unsual background for a Senate candidate anywhere. He is a former antiwar activist, social worker, and school teacher who served as a Senate aide to Walter F. Mondale, and acting commissioner of Economic Development in Minnesota.

Durenberger, who holds Hubert H. Humphrey's old Senate seat, has mounted the second most costly campaign in Minnesota history, raising $2.7 million. But he seems at a loss on how to deal with Dayton, who many GOP observers feel has peaked in the race.

The senator and his staff harbor a deep-seated bitterness against Dayton. Durenberger privately describes his opponent as "a myth," an inexperienced neophyte who "can spend $6 million making himself look like the Second Coming."

But Durenberger is reluctant to air these feelings in public.

"I'm trapped. I'm the high-road nice guy. I can't attack him directly," he said in an interview here. "But I can't let them take a guy who is a nothing and let him beat me."

Frenzel, who represents a suburban Minneapolis district, is playing "the heavy" for Durenberger. Twice in recent weeks, he has called press conferences to question Dayton's credentials. Several attacks have been false. Others have been either distortions or just plain silly.

He incorrectly charged, for example, that Dayton, a Yale graduate who once taught in New York City slums, never had a teaching certificate. He also implied that the only way Dayton got a job with a group working with runaway boys in South Boston was by donating money (about $30,000 a year) to the organization.

Dayton, however, has been caught in at least one embarrassing incident of his own: a clumsy attempt to win the endorsement of the National Rifle Association, which upset many liberals.

Dayton's money isn't Durenberger's only problem.

Minnesota, like the rest of the Midwest, is an economic disaster area. Though the state's unemployment rate is 7.3 percent, well below the national average, it exceeds 40 percent in some areas, and many farmers are caught between high interest rates and low grain prices.

"If everything were going well, Dave would be safe from any challenger, save Fritz Mondale," said Dayton, who is making the race a referendum on Reaganomics.

This puts Durenberger on the defensive. He is a genuinely likeable campaigner, well-respected among his Senate colleagues.

Durenberger, who must attract widespread support among Democrats to win, has spent much of the last two years distancing himself from Reagan. He has called Reagan's arguments for New Federalism "baloney." And when the president was greeted by protesters during a Minnesota visit, Durenberger said if he weren't a senator, "I'd be out there demonstrating myself."

And yet, with the election less than two weeks away, Durenberger is now embracing Reaganomics, urging voters to "stay the course" with the president.

"When will it the economy turn around? When will I get my job back? When will housing be affordable?" he asked a breakfast meeting at Kahler Restaurant here. "I can't tell you, gang. Because I don't know. But it's coming. Yes, we're on the right track."

Nonetheless Republicans in this southern Minnesota city are worried about their senator.

"It's going to be a very close race," State Rep. Bob Haukoos said before a Durenberger breakfast the other day. "I think Reagan is still popular here. But his administration is a little different thing."

"People still vote their pocketbooks. Durenberger is a Republican and that will hurt him," added Kent Erlandson, a John Deere farm implement dealer. "Reaganomics all depends on what side of the fence you're standing on. It sure hasn't worked for farmers around here."