When an ambitious Democrat here named Joe Timilty signed up early in 1976 to help a long-short presidential candidate named Jimmy Carter, the kind of people who tend to be suspicious of anything a politician does -- that is, other politicians -- suspected that Timilty had an ulterior motive.

Timilty had a yearning to unseat Boston's veteran Mayor Kevin White. Politicians here thought Timilty's presidential choice was a calculated gamble: If this Carter could win the White House, maybe he could help Timilty win City Hall in the 1979 election.

What happened? Carter won. Timilty has run. And this fall, all signs indicate that Joe Timilty will be beaten, as he has been twice before, by White.

White, whose 12 years in office make him the nation's senior big-city mayor, is a strong favorite to win a new four-year term, partly because of his considerable success at building a personal power base and partly because his opposition, while virulent, is fragmented.

White and three other Democrats are competing in a "nonpartisan" race that seems to have aroused even less interest than the 1975 contest, when fewer than 15 percent of Boston's adults turned out to vote. In a preliminary election today, the field will be reduced to two finalists who will meet in a November runoff.

All handicappers agree that White will lead the field today. Timilty, a back-slapping, glad-handing state senator who has a loyal corps of supporters in the Irish neighborhoods, seems likely to run second, earning another head-to-head race with White.

If that happens, it is hard to see what Timilty could do to reverse the 1975 result. The Carter connection seems to have done him no good, and he has failed to stir up excitement in an electorate that is busy concentrating on two Polish heroes, Carl Yastrzemski of the Red Sox and Pope Paul II, who will say a mass on Boston Common next week.

White backers would be more worried if one of the other challengers made it to the final election. They say either Dave Finnegan, a friendly Irishman who is considered a "new face" despite three terms on the school board, or Mel King, a brooding black who appeals to minority groups and rich, white liberals, whould have a better chance of putting together a coalition that could topple White.

Any challenger's chances, though, look dim.

White, who celebrates his 50th birthday today, has managed to gain support among all segments of this urban melting pot. His determined efforts to make school busing work seem to assure him of most of the black vote. His work in revitalizing the downtown section makes him popular with business. His skill at spreading around city jobs and federal money has won him backers in every neighborhood.

On the other hand, taxes are up, and the quality of services like housing and health care is down. And White, like many entrenched politicans, is not averse to using the perquistes of his office to enhance his political status.

When Finnegan passed out campaign fliers in City Hall, the mayor's office ordered them confiscated and destroyed. But when White held a fund-raiser last week, his own campaign staff fanned through the building and "invited" city employes to make contributions -- voluntary, of course -- of $50 each.

The most common explanation of such things is that Kevin White wants to build a machine like the one that sustained Mayor Richard Daley for years in Chicago. White's backers don't object to the comparison. "You know, White has made this city work just like Daley did," boasts George Regan, an top assistant.

As Daley proved, it takes heavy artillery to dislodge a big-city machine, and some people here thought the Carter White House might fill that role for Joe Timilty.

Timilty himself has only pleasant words for the president, but his campaign workers are bitter about what the White House has not done.

They say that after the 1976 campaign -- where Timilty ran Carter's Pennsylvania operation -- Timilty had hoped for a sub-Cabinet job in some agency dealing with cities. Instead, he got a part-time spot on an obscure federal commission; the commission issued a report that was duly praised by Carter and has since been duly ignored.

In the campaign, Carter aides raised a few thousand dollars for Timilty, but didn't come close to the $100,000 that Timilty's treasurer had hoped they would generate. And Carter's weak image seems to have turned into an asset for none other than Kevin White.

As White's campaign staff explains it, the perception that Carter as a pantywaist executive has made the voters anxious for a tough administrator -- even, perhaps, for a machine boss and that's an image that White seems anxious to convey.