Two former national party chairmen have suggested that the political parties shorten their 1992 national conventions and try to trade the unused television time in the summer for blocks of free network time for their candidates during the fall campaign.

Former Democratic national chairman Charles T. Manatt and former Republican national chairman Frank J. Fahrenkopf headed a Center for Democracy panel that yesterday suggested giving up one of the four nights of prime-time convention coverage and requesting the networks in return to offer three hours of free prime-time broadcasts to party nominees in September and October.

The report acknowledged that conventions have become less compelling to a mass audience since the primaries have become the decision-point for settling the presidential nomination. But it argued that the quadrennial gatherings still are vital for the parties.

The commission suggested that the free time be offered simultaneously on CBS, NBC, ABC and CNN and that it be given in half-hour blocks, back to back, in September and early and late October, to be used only for live broadcasts with the presidential and vice presidential nominees. The goal, Manatt and Fahrenkopf said, is to "increase the substance" in the campaign at the time the voters are most interested.

A problem: The networks already have indicated they plan to cut back the hours they devote to live coverage of the 1992 conventions.

A bigger problem: The networks have fiercely resisted making free time available to candidates in formats the networks do not control.