The government's planned prosecution of three union presidents for alleged violations of the Hatch Act is a "silly" and "distorted" application of the 1939 law and contradicts previous enforcement policies, Rep. Patricia Schroeder (D-Colo.) said yesterday.

Schroeder said the proposed prosecution of federal workers on long-term leave may be "technically" correct, but "it is silly to conclude this is what Congress meant in passing the law."

"The whole intent of the act is to prevent political coercion in the work place. If you are a federal employe taking leave without pay, how do you engage in coercion in your work place?" asked Schroeder, chairman of the House Civil Service subcommittee that oversees Hatch Act enforcement.

The union officials have been on leave without pay from their federal jobs for at least 14 years. The planned prosecution by the Office of Special Counsel marks the first time the act has been applied to employes on leave for activities connected to union duties, according to government lawyers.

The act prohibits federal workers from campaigning, seeking office, raising funds, distributing literature or coercing other employes for political purposes.

The three officials, Kenneth T. Blaylock of the American Federation of Government Employes, Moe Biller of the American Postal Workers Union and Vincent Sombrotto of the National Association of Letter Carriers, were told Monday that they will be prosecuted for campaigning on behalf of Democratic presidential nominee Walter F. Mondale unless they resign their federal jobs or retire by Feb. 26. All three said they would fight the case in court.

K. William O'Connor, the special counsel responsible for enforcing the law, yesterday called his actions "apolitical" and consistent with his enforcement policies. The letters to the three officials are warnings of prosecution, he said.

He also said Congress has never amended the law to exempt employes taking unpaid leave or carrying out union activities.

The basis for the prosecution, according to sources in the Merit Systems Protection Board, will be the union presidents' columns in their union publications that urged members to support Mondale and criticized President Reagan.

O'Connor said at 1983 hearings that he preferred to warn violators of the Hatch Act to stop their actions rather than prosecuting them. "I believe more effective results can be achieved by prophylaxis than prosecution," he said.

O'Connor's office has prosecuted five Hatch Act cases in his 29 months in office. He testified that prosecutions are costly and should be limited to "significant" cases.

Sen. Thomas F. Eagleton (D-Mo.) called the action "rank hypocrisy" by the Reagan administration, because officials took no steps to discourage 21 U.S. ambassadors last year when they endorsed Sen. Jesse Helms (R-S.C.). Eagleton said he will introduce a bill to include ambassadors, now exempt from most of the Hatch Act, in that law.