The lawsuit the U.S. government filed this week in an effort to take over the Teamsters is an unprecedentedly drastic and potentially dangerous step.

But then sweeping action is warranted because the nation's biggest union also seems to be its dirtiest, an embarrassment to the rest of the labor movement that erodes public confidence in organized labor.The lawsuit is the first attempt by the government to use the federal Racketeering Influenced and Corrupt Organizations Law to put an entire international union under trusteeship until free elections can be held.

The corruption goes so deep that free elections alone may not do the job. Delegates to past Teamster conventions have been known to cheer the union's leaders even after those leaders were sentenced to prison.

Under the circumstances, it's easy to understand why government officials think a trustee, if granted by the courts, could be in place at the Teamsters not just for months but for years.

So far, four of the last five Teamster presidents have been indicted on criminal charges. Three of them went to prison and a trial is pending against the current president, Jackie Presser.

Plenty of lesser Teamster officials also have been linked with organized crime over the years. Since 1970, the government has convicted more than 300 people connected with the Teamsters, including many union officials, for such crimes as extortion, bribery, and embezzlement of union funds.

No wonder that a 1986 presidential commission concluded that the Teamsters Union has been "firmly under the influence of organized crime since the 1950's."

No wonder, too, that Reuter News Services reports the Teamsters have been investigated more intensively than any other union in American history.

Though a few Teamsters have occasionally tried to clean up the union, the reform efforts got nowhere, thwarted either by threats or stony indifference.

Even their ouster from the AFL-CIO some 30 years ago didn't chasten the Teamsters. Nor has the AFL-CIO made even a stab at cleaning house since readmitting the Teamsters last October.

Ordinarily, a government takeover of a union ought to be opposed as a threat to freedom. But in the case of the Teamsters, Washington's historic move this week simply represents a belated effort to do a job that the unions themselves should have done long ago.