In what could be the ultimate test of their influence, federal and postal union leaders have directed their million-plus members to do almost anything today -- rake leaves, talk to the spouse or tune in "Masterpiece Theater" -- other than watch pro football if the strike continues.

The participation rate in the proposed federal union boycott of National Football League games will never be known, because members will comply, or not, in the privacy of their own homes. But that their union leaders are seriously pressing the don't-watch-scabs campaign, which is rather remote from civil service matters, illustrates a new effort to build bridges with other unions and make federal dues payers feel more like trade unionists.

Traditionally, federal and postal unions confined their activities to representing members at grievances, negotiating basic work rules and lobbying Capitol Hill for pay raises and improved fringe benefits. But in recent years, many of the unions have spent more time getting involved in political and social issues.

In the past 10 years, some government unions have told members not to buy products -- from grapes and lettuce to South African gold coins. They have urged members to demonstrate against certain foreign policy initiatives and boycott the government's in-house charity fund-raising campaign. Federal and postal union leaders have joined entertainment and sports celebrities at rallies concerning social issues and become involved in political races -- from the White House to Guam and the District Building.

The strike by the National Football League Players Association gave government union chiefs the chance to use their union credentials in a high-profile strike that has attracted nationwide attention. Last week, groups of Washington-based union leaders picketed RFK Stadium here in a show of solidarity. Today, union leaders from Atlanta to Seattle will be marching alongside striking Minnesota Vikings and Cincinnati Bengals outside stadiums where nonunion games are to be played.

Most unions have sent letters of support to the players union, and of nonsupport to football team owners. Union bulletin boards at federal agencies and post offices are crammed with "Don't watch scab games" messages.

The American Postal Workers Union has assigned its number two officer, William Burrus, to coordinate activities with the striking players, and union President Moe Biller has pledged the support of the union's 335,000 members to the strikers.

The most recent bulletin of the National Association of Letter Carriers union is devoted almost entirely to the football strike. Union President Vincent Sombrotto told his members, whose average pay is about $27,000, that the high salaries of the football players should not distract federal workers from the fact that the players typically have short careers and are sometimes treated more like property than people.

"This may seem remote from our traditional mainstream," a union official said Friday, "but our unqualified support of the players is serious . . . is not a public relations stunt and will pay off. The next time we have a serious beef with the government, imagine the public relations value of having a 300-pound, household-word football player standing alongside us, or testifying with us on Capitol Hill."