"JUST WAIT," Frank Rizzo promised his opposition, in the course of campaigning for mayor of Philadelphia. "I'm going to make Attila the Hun look like a faggot." He wasn't kidding. Mr. Rizzo has not been what you would call conscientious about most of his campaign promises. It may even be arguable (depending on your view of Attila) whether he has entirely made good on that one. But this much you have to give him: He has never stopped trying.

He has called for the revival of the electric chair and expressed a yearning - personally - to "pull the switch if they run out of people who want to do it." (He has not, however, been picky about means of execution; of the Black Panthers he once said: "They should be strung up - I mean withing the law.") The "way to treat criminals," he said, "is scappo il cappo " - break the head.

In short, he has well earned his reputations as a hamhanded, head-bashing, race-baiting champion of "white ethnics." Relentless in his repression of civil liberties, serene in his indifference to due process, tireless in his suspect manipulation of political machinery, he has done more than a make a mockery of Brotherly Love. He has become a national embarrassment.

Last April, the Ku Klux Klan was sufficiently impressed to award him the title "Racist Hero of the Month." Somehow offended, he rejected it. But when he launched a campaign this fall for a charter amendment to permit him to run for an unprecedented third term in 1980, his campaign slogan was eloquent in its simplicity: "Vote White."

Last Tuesday, the charter amendment was on the Philadelphia ballot, and Frank Rizzo ws still hard it, polishing his Attila image. We are still a little uncertain about all the electoral outrages commited in his behalf, but the word "steal" seems to describe pretty much what is was all about. Voting machines broke down - in black precincts. The hands, for votes. The head of the election commission, a Rizzo supporter, was arrested. Before the day was over, judges were busy ordering compliance with election rules, and the FBI was out in force, issuing subpoenas. A grand jury has already been convened.

And the truly wondrous thing about it is that it didn't work. It wasn't even cloke. An extraordinary coalition of fed-up Philadelphia businessmen, bankers, lawyers, church groups, black organizations and newspapers - including the American Jewish Committee, the Chamber of Commerce, the Americans for Democratic Action, the Women's International League for Peace and Freedom and the Gay Alliance - came together to repel Mr. Rizzo's grab for a third term. The turnout was stunning: There were some 20,000 more votes in Philadelphia on the charter question than in the choice for governor.

There may have been grander victories - or more satisfying defeats - last Tuesday as Americans attended to the routine business of filling congressiional seats or venting their yearnings for tax relief. But we can think of none more reassuring to the cause of decent government than Philadelphia's decision, by a resounding 2-to-1 vote, to put a final deadline on the shameful reign of Frank Rizzo.