Sen. Alan Cranston (D-Calif.) says that if he is elected president he would favor making government workers who do not belong to unions pay the equivalent of union dues to labor organizations that represent them.

Cranston gave his strong support for the "agency shop" in government on a videotape featuring five of the six announced Democratic presidential candidates. The American Federation of Government Employes union is showing the tape to its members nationwide.

Although the government does not have data on how many employes are union members, the Office of Personnel Management says that as of March 1981 slightly more than 1.2 million of the government's 2.6 million workers were in so-called exclusive bargaining units represented by unions. Actual membership is much smaller because many workers who are in those units are not union members.

Reporters were shown the videotape last week at the headquarters of the AFGE, which, with about 250,000 members, is the largest federal union.

The tape also features statements from Sens. John Glenn (Ohio), Ernest Hollings (S.C.), and Gary Hart (Colo.), and former vice president Walter Mondale. Because of scheduling problems, the other announced Democratic candidate, former Florida Governor Reuben Askew, does not appear on the tape.

The candidates, given the questions beforehand, were interviewed by Jane McMichael, who heads the AFL-CIO union's legislative-political action department.

Glenn, Mondale and Hart did not respond directly to the question of the agency shop, although all three said they support greater collective-bargaining rights for federal workers, along with improved pay and a solid retirement system.

Hollings said he opposes the right of government workers to strike, but said he would favor arbitration to settle disputes between the government and employe unions.

Federal unions (except in the Postal Service) cannot bargain over wages or fringe benefits, which are set by the president and Congress. But they represent employes in grievances, and take an active role in contracts covering, among other things, day care and some working conditions.

The two most heavily unionized federal agencies are the Postal Service (about 80 percent of the rank-and-file workers belong to unions) and the Tennessee Valley Authority, both quasigovernment corporations.

Union membership rarely approaches 50 percent in other agencies of the government. But unions do represent employes (whether members or not) and about half the nonsupervisory work force is in a bargaining unit covered by an exclusive union contract.

Virtually all government agencies have the so-called "open shop" rule, which permits but does not require workers to join unions.

In response to a question about collective bargaining in government, Cranston said:

"I think there should be an opportunity for collective bargaining. There should not be an open shop arrangement. There should be a full and fair opportunity for people who are working for the government to choose leaders who can negotiate with the government on the conditions of their service, the pay they receive and all other aspects of their employment."

AFGE has one of the only agency-shop agreements in the government. Its arragement with the D.C. Government provides that employes in units with 60 percent union membership are required to pay a "service fee"--the equivalent of dues.

Cranston's strong support of the agency shop has boosted his standing with AFGE leaders who have seen the videotape. The union, which is preparing its endorsement in conjunction with the AFL-CIO, plans to show the tape to all of its 1,300 locals around the nation.

In some southern states with big government and defense installations, AFGE is the largest union. So its backing could be critical in the delegate selection process.

Better-known candidates Mondale and Glenn got the most support from AFGE union leaders in Florida after the tape was shown. Cranston came in third.

In North Carolina, leaders split between Mondale and Cranston. Mondale won the most AFGE leader votes in Iowa where President Reagan and Mickey Mouse each got one vote.