In Cambridge these days, running against Joe Kennedy is an exercise in media obscurity. As the 12 candidates running for the seat of retiring House Speaker Thomas P. (Tip) O'Neill Jr. gathered for the first so-called debate of the campaign, TV cameras recorded Joe's every move. The others had plenty of time to talk to themselves.

The Boston Herald blazes Kennedy's comments on Page 1. The only way the rest get into print is to comment on what Joe said. The Boston Globe says it is trying to give equal time to everyone, but that has meant so little play per candidate that one contender complains the paper has put itself "outta the news business."

Politicos are betting on three going the distance besides Kennedy -- State Sen. George Bachrach, whose archliberal constituents make up 30 percent of the district; Mel King, the black activist who did well in the mayoral race; and state Rep. Tom Vallely.

The artful King needs to pull from liberal ranks in a district only 3 percent black. He is the only major candidate to announce after Kennedy and loftily chooses to ignore him. "As the campaign unfolds, each of us is going to have to put it out there in terms of issues," he says.

Vallely and Bachrach are also running on their records, but they decided to hit Kennedy head on. Bachrach styles himself the candidate "for the rest of us." Vallely takes the Avis approach -- billing himself as "the other guy." Against Bachrach, Vallely says ruefully, "I would not have to be 'the other guy.' I'd be the leader."

One of the few to play light with his biography ("He mastered high school biology in just four years and never did figure out Spanish"), Vallely won the Silver Star in Vietnam, came back to march with Vietnam Veterans Against the War and succeeded Barney Frank in the state legislature in 1980.

One of Vallely's campaign films is a documentary of him in Vietnam made last year. Some of his opponents scoff that he overemphasizes his Vietnam experience. Says Vallely, "I have no intention of not being myself, and Vietnam is a lot of me. I learned a little bit about foreign policy getting shot at in Vietnam."

Vallely jokes that Kennedy is "not your average Joe," but Bachrach pointedly says, "Anyone who thought he had big bucks going for him is now in trouble. Joe's got bigger bucks."

Vallely is not only "the other guy," he is "the other millionaire." He acknowledges that his wife's holdings make him a wealthy man, but quips that, compared with Kennedy, he is "just a neighborhood kid."

Bachrach is one of the least affluent and likes to say so. He thinks his district supports him because "I'm one of them. They know I've worked hard in the peace movement, environmental and women's issues. Joe's not been there shoulder-to-shoulder with them."

In a race estimated to cost in the $750,000 to $1 million range, money may be the deciding factor in forcing other candidates to call it quits. One other famous name, James Roosevelt, officially launched his long unofficial campaign yesterday and has holdings worth half a million. So far, however, being the grandson of Franklin D. Roosevelt seems to have cut little ice with constituents.

Down in Baltimore County, it is a different story. Don Hutchinson, local leader of the Democratic Party, was supposed to challenge the Republican incumbent, Rep. Helen Bentley, but decided to run for the Senate. Kennedy's sister, Kathleen Kennedy Townsend, says, "All the other elected officials would be giving up a seat for a very tough campaign. So I figured, well, I'll try. I don't think I could win the party nomination very easily if there were all sorts of other viable candidates."

Bentley, recovering from a kidney operation, says she sees no threat from Townsend. who can do things for them. I have a very excellent constituency record." The nuts-and-bolts congresswoman promises this contest could get fiery if not downright ugly. Bentley says a whisper campaign was started that "I was very sick and would have to withdraw from the race. I'm sure that came from the enemy camp."

Says Townsend: "I'm not that enemy. In fact, I sent her flowers."