Democratic and Republican party leaders, here this weekend to find ways to strengthen their parties, agreed to cooperate in ending most of the nation's "open primaries." But in the effort to curb the proliferation of primaries, they were unwilling to push unpopular rules on their respective parties.

"This is the first time the two parties have jointly sponsored anything," said Ronald H. Brown, deputy chairman of the Democratic National Committee. "It's incredible that here are leaders of the two major parties who have never met before or been in the same room together."

The 50 party leaders and academics who met at Harvard's Institute of Politics agreed the "open primary" system, by which individuals can vote in the other party's primary, should be abolished.

They adopted a resolution to encourage restricting voting in a party's primary to its members. Afterward, in an interview, Roger Allan Moore, general counsel of the Republican National Committee, said, "I think we will cooperate with the Democratic Party to close the open primaries, except in Wisconsin and in some southern states like Alabama."

But in general, those meeting here cautioned against attempts to impose change on state parties by national committees or state and federal legislatures.

"Nobody has suggested a cure which I don't regard as worse than the disease," said Roger Allan Moore, general counsel of the Republican National Committee. "One thing that was unanimous was that there should be no congressional action . . . no tampering."

"Half of the new generation of voters declare themselves 'independent,' " said North Carolina Gov. James B. Hunt Jr., chairman of the Democratic Commission on Presidential Nomination, which the Democrats have set up to devise changes in their presidential nominating procedures. "The nomination process contributes to the fragmentation of our parties.

"We see executives and legislators alike thinking 'I got here myself,' " he said in a speech alluding to Jimmy Carter's rise to power through media-based primary election victories, often independent of local party leaders.

"We agree that the role of the parties has been seriously diminished in recent years," said Ernest Angelo Jr., chairman of the RNC Committee to Study Electoral Reform. "But we question the assumption that most of this weakness is due to the nominating process." Having gained control of the Senate and the White House, Republicans here were less inclined to tinker with the system.

The major problems raised at the conference:

The trend away from party caucus selection of candidates to primary elections, bypassing the party leaders.

The primary and caucus "open window" by which early New Hampshire and Iowa candidate selections influence campaigns in other states, prompting long, costly campaigns.

The failure of congressmen and other party leaders to attend nominating conventions that have been dubbed "rubber stamps" because delegates are bound by results of primary elections.

A paper adopted at the conference recommended "no national primary" and "no national imposition or mandate by Congress or a national party," reflecting a strong undercurrent here against forcing state parties to accept unwanted changes.