PORT HURON, MICH. -- Warren Brewster sells fruits and vegetables out of a truck across from the Mueller Brass Mill where he worked for 30 years. A swing voter, he has cast ballots for Republicans Ronald Reagan and George Bush for president as well as for Democrat James J. Blanchard for governor.

This year Blanchard is running for a third term and Brewster, 67, doesn't have a clue as to what he's going to do. "I'm not saying who I'm going to vote for, because I don't know," he said on a recent sunny day as he peddled fresh squash, tomatoes and pumpkins to passing motorists. "I've got to see who tells the best lies."

Skeptical, even cynical, voters like Brewster hold the key to Blanchard's prospects in November. As candidates across the country prepare for a potential onslaught of anti-incumbent sentiment this fall, the incumbent Democrat and his Republican opponent, John Engler, are struggling to judge its effect here.

Engler, the state Senate majority leader who has served in Lansing for 20 years, is hardly a fresh face, and polls here show that he has not yet benefited from any of the discomfort some voters profess to have for Blanchard. A statewide survey released last week by the Detroit Free-Press and WXYZ-TV showed Blanchard widening his lead over Engler to 20 percentage points, up eight points since August.

"An outsider who never served in government who had no experience would be a stronger threat than Engler, who's been a part of the capital crowd longer than I have," said Blanchard.

But Engler, 41, who is hammering away at Blanchard's record on taxes and education spending, predicts voters will make a distinction between an incumbent executive and an incumbent legislator who represents a different party and a different approach to government.

"There's a real mood out there that things aren't working well," he said.

Blanchard, 48, who trounced Republican William Lucas to win his second term in 1986 and emerged in national political circles as part of a generation of bright, young state executives, seems to sense the unease that other seemingly entrenched incumbents are also detecting. But in this atmosphere, Blanchard argues that incumbency can be turned into a strength.

"With the Middle East crisis and the economy failing, people are nervous," said Blanchard as he prepared to embark on a Dearborn Heights neighborhood walk. "That's a plus. People view me as a good economic manager."

Blanchard earned his reputation by helping to steer the Chrysler loan guarantee through Congress at the end of his second term in the House in 1982. A modest economic revival in a state that has been heavily dependent on blue-collar industry has also aided Blanchard, who said Michigan voters "will want to keep the leaders who were tested in the toughest of times."

But Blanchard has been forced to recover from several of the early miscues that plagued his campaign. The first occurred when Blanchard's campaign ran a television ad on the creation of boot camps for prison inmates that was assailed as racist because it prominently featured blacks in the role of prisoners. In spite of the criticism, Blanchard said the ad ultimately helped more than it hurt among voters concerned about crime.

Then Blanchard's ex-wife Paula published a book, "Til Politics Do Us Part," that accused the governor of having had an extramarital affair with the woman who is now his second wife. Finally last month, Blanchard was nearly overwhelmed by an avalanche of negative national publicity when he dropped his elderly lieutenant governor, Martha Griffiths, from the Democratic ticket.

"He hasn't had two or three straight weeks where he has been able to control the agenda," said Tom Shields, a Lansing GOP consultant who is working for Engler. "None of these are the silver bullet, the smoking gun that's going to kill him. The cumulative effect is what's going to do it."

Engler, to no one's surprise, chooses to emphasize the down side of the Michigan story. "Michigan is a comeback state; that's a given," Engler told a lunchtime meeting of the Port Huron Optimists and Rotary last week. "So are the other 49 states. The story is, how does our comeback story stack up?"

At a distance, the two men are cordial toward one another, but this is not the relationship voters are seeing in the battle of television commercials. One Blanchard ad, paid for by the Michigan Democratic Party, takes cameras to the streets of Engler's home town of Mount Pleasant to ask people on the street -- including the secretary of the local Democratic Party -- what they think of Engler. None of the responses is flattering.

For his part, Engler has run an ad charging that "tricky bookkeeping" has kept profits from the state-run lottery from being used for education. He does not mention that he presided over the state Senate that approved the budget decisions he now criticizes.

Blanchard said he expects the race to become closer as voters begin to make up their minds. But Michigan's political climate, he maintains, has little in common with Massachusetts and Oklahoma, where anti-incumbency sentiment among voters has been most evident.

"It depends on the incumbent," said Blanchard. "I don't see it as much of a problem for us."

Recently, Blanchard received support for this view from an unlikely source. The 78-year-old Griffiths, who was his loudest critic when she was unceremoniously dumped from the ticket last month and replaced with a younger woman with similar credentials, now predicts Blanchard will defeat Engler, but not easily.

"I think it will be a tough election," she said. "It'll be hard. He's had two fairly easy ones, and I think that's all you're entitled to, really."