On his weekends, Mayor William Donald Schaefer likes nothing better than to cruise the alleys and backstreets of his aged port town, hunting for gaping potholes and messy trash piles that escape the attention of city workers.

Sometimes, he hops out of his car and peeks over the backyard fences of the tiny, cluttered rowhouses here, jotting down addresses when he spots a neat garden patch or fine shrub that might deserve one of his special commendations, "The Order of the Red Rose."

These curious scouting missions by the major of America's eighth largest city are open to sharply different interpretations of Schaefer, a 58-year-old balding bachelor who lives with his mother Tululu in the simple brick town house of his childhood.

To his backers, the off-hours forays are just another example of Schaefer's indomitable dedication, a rare political energy that has helped transform Baltimore from the blighted, depressed town of the 1960s into a national symbol of urban renaissance today.

To his few detractors, the same zeal that sends Schaefer into the forgotten corners of Baltimore has turned him into the "imperial mayor" who sputters at opposing views, browbeats aides, places reporters on "suspension" for unflattering stories and ignores the city's poor in favor of glamorous building projects.

Any debate on Schaefer quickly ends at a certain point. No one disagrees that this improbable politician who resembles a jolly German innkeeper has so injected himself into the bloodstream of Baltimore as to make his own career and the sity's success all but indistinguishable.

In the process, he has achieved something few big city mayors -- especially white mayors in predominantly black cities -- can claim. He has neutralized all meaningful political opposition in his first two terms and governs with almost unquestioned authority as he seeks his third term this fall.

Nothing better illustrates Schaefer's immense popularity than the quality of opposition in Tuesday's Democratic primary. His most formidable challenger is a little-known freshman member of the State legislature who gives himself at best 1-in-5 odds of winning.

"Don Schaefer has convinced everybody that he is the city," said City Council President Walter S. Orlinsky, the mayor's peskiest critic. "An attack on Schaefer is an attack on Baltimore. Nobody wants to be unpatriotic, so nobody takes him on."

Schaefer's rise as the political monarch of Baltimore parallels the resurgence of the city itself, a career built brick by brick like the classy skyline that skirts the Inner Harbor and welcomes visitors driving north from Washington 45 miles away.

Indeed, the graceful steel and glass structures embellishing the once shabby waterfront of rotting wharves and warehouses serve as looming campaign posters for Schaefer, who dedicated each building with fanfare, even though he merely implemented the plans of previous mayors.

The revitalized neighborhoods also stand as monuments to Schaefer: cheerful Old Towne Mall sitting in the commercial sector most savagely hit by the 1968 riots; the charming Stirling Street corridor reclaimed by the $1-a-house "urban homesteading" program; and the imaginative Coldspring housing built on a steep and rocky cliff by Moshe Sadie, an internationally-known architect.

The vitality of "new" Baltimore not only reflects well on city planners. It is evidence of Schaefer's uncanny ability to attract the kind of capital -- first public and more recently private -- necessary to turn his grim industrial town into an all-American city.

In the early years, he ventured forth to Washington, sheaths of paper stuffed under each arm, pleading for government investments in Baltimore's future. His housing commissioner then, Robert C. Embry Jr., who is now a high-ranking federal housing official, became an expert in grantmanship, applying for every available source of funds until Baltimore was one of the largest recipients of federal aid.

Schaefer has been equally dogged in pursuing private channels. When a New York bank pulled back an informal commitment for a $20 million mortgage loan to build a Hyatt Hotel in the Inner Harbor, he did not rest until he convinced the bank president to visit Baltimore and reconsider the decision. The president cleared the way for the bank loan and pledged another $50 million for future city projects.

"His enthusiasm is infectious," said William Boucher III, head of the Greater Baltimore Committee of businessmen and professionals. "He goes out and sells the city as the best in the world and gets others to believe him."

For Schaefer, the city's now famous face lift has done much more than remind the rest of the world that Baltimore "is not that little place between Washington and Philadelphia." The new look, he says, has instilled civic pride in the city's 850,000 residents.

"People were very skeptical at first," he recalled in an interview. "I wanted to show them the city was going some place, so I had signs put up every time we started a new project. As people saw how much was going on they decided this isn't such a bad place."

This public consciousness-raising -- convincing the once, wary taxpayers that, as the mayor never tires of saying, "Baltimore is best" -- forms the essence of the Schaefer years and the basis of his popularity, according to political and business leaders here.

When he is not roaming the alleys in search of trash piles, Schaefer can be found sitting in the back row of neighborhood improvement association meetings scribbling notes on his ubiquitous "action memos," chomping on a Polish sausage at a church bazaar, rummaging through used books at a street fair, calming merchants whose stores were vandalized or officiating soapbox derbies in city schools.

As he hops from community to community in this ethnically balkanized city, he comes across as the quintessential ombudsman and father confessor, rolling his ample jowls into his familiar "how'r'ya" greeting, furrowing his protruding brow when presented with personal problems, always scribbling notes.

Baltimore has no better salesman. Here he is pinning a "Baltimore Is Best" button on President and Mrs. Carter when they visited the city last month. There he is decorating a young man who planted a "Baltimore Is Best" banner on the summit of Mount McKinley.

Anthony (Peewee) D'Angelo, a city worker, summed up Schaefer's role at a rally for the mayor at Bud's Crabhouse last week. "This man is married to the city of Baltimore and all of us here."

The geniality Schaefer exudes in public dissolves quickly in private, creating what a City Hall regular calls "the eternal contradiction of Don Schaefer." A temperamental, demanding and proud man, he enjoys calling up his sides "just to ride their ass a little" and denouncing them for mistakes in the presence of colleagues.

For a politician courted by presidents, governors and corporation chiefs, he has maintained a disarming simplicity. Visitors to his office are asked to sit still while he snaps their photograph will his handy Instamatic. He relaxes by watering a virtual forest of African violets near his desk. His favorite breakfast is Egg McMuffin from McDonald's.And he vacations in a house trailer in Ocean City where he fishes off the pier.

His relation with the City Council generally are good, primarily because he almost always gets his way. Treating the 19 council members like potentially naughty children, he rewards one of the staunchest allies with strawberry pies from Hausner's restaurant.

When the council rebuffs him, he lashes out, often punishing his opponents. Since Council President Orlinsky assumed the lonely role of Schaefer critic four years ago, the mayor has refused to talk to him and has attempted to shut him out of city functions whenever possible. After the council tried to cut his budget in 1976, Schaefer ordered his department heads to channel through his office all requests made by council members on behalf of their constituents.

"The mayor gets burped wrong sometimes," said Councilman Dominic Mimi DiPietro, "and he stays burped wrong all day."

Council members are not alone in feeling the Schaefer wrath. After the Greater Baltimore Committee's Boucher disagreed with the mayor over the city's downtown retail district revitalization program, Schaefer refused to acknowledge him for several months. Then, he accepted an invitation to the group's annual dinner and never showed up.

Among Schaefer's little demons, none troubles him as often and as deeply as the Baltimore press corps, which he regularly finds guilty of "negativism" and "sensationalism." Few weeks go by when the mayor does not call up newspaper editors to complain about stories and he punishes particular offenders by placing them on "suspension" for several days or weeks, depending on their infraction.

When he prefers to avoid questions, he simply makes himself inaccessible or responds with a rapid fire chorus of "What? What? What? What?, " spitting out the replies like a quacking duck.

In recent weeks, Schaefer consistently has refused to discuss his campaign with reporters, threatening to walk out of press conferences if questions persisted. Finally, he angrily consented to "talk politics" late last month, beginning the session by reading off a long list of his administration's accomplishments for 25 minutes. When he agreed to take questions, he stared straight ahead at a blank wall avoiding the eyes of reporters.

Schaefer's associates plain his exasperation in simple terms: After eight years as mayor, he has successfully controlled and to some extent cowed every interest group in the city save one -- the press.

The Baltimore Sun endorsed Schaefer for a third term recently, praising him for the "splendid things" he has done for the city. But the newspaper reiterated a criticism that infuriates the mayor, accusing him of ignoring "the large number of city residents who are ill-housed, ill-fed and ill-starred."

The argument is made in other quarters as well that the Baltimore of Mayor Schaefer is the Baltimore of the wealthy, that he has simply created "two Baltimores" -- one for the well-heeled and one for the poor people who remain crammed into unsanitary, dingy houses.

In fact, Baltimore has one of the highest percentages of welfare recipients in the country and a large number of city residents are abysmally poor. One of every three families lives in a house that fails to meet housing code standards, according to a recent news report, and 24,000 families remain on a waiting list for public housing.

"The mayor represents about 30 percent of the people," said Bob Cheeks, executive director of Baltimore's Welfare Rights Organization. "His priority is structures and buildings for the middle classes.

"What does a poor man care about a convention center or a skyscraper if he can't afford to go there?'

Schaefer bristles at such criticism, insisting that the city spends $8 of every $10 on social services. He cites his administration's free food programs and neighborhood revitalization efforts as "proof that we do more for the poor than any city."

If Schaefer seems sensitive about the poverty issue, it is not because he feels politically vulnerable. Even in the large and poor black community of Baltimore -- blacks make up 56 percent of the city's population -- the mayor appears unbeatable. The polls show he is nearly as popular among blacks as he is among whites.

How long Schaefer remains mayor may have less to do with his own ambitions than the racial balance of Baltimore and the organizational efforts of the city's fractious black politicians. In any case, Schaefer is widely considered to be Baltimore's last white mayor.

"The blacks aren't ready now," said former Mayor Thomas J. D'Alesandro III, who handed the city to Schaefer in 1971. "The blacks can live with him for the time being and the whites see him as the last white mayor. With those ground rules you can do a lot of tap dancing. Everybody's off balance.

"I don't know if that'll be true four years from now."