Chairman Alan Greenspan yesterday postponed today's scheduled session of the deadlocked National Commission on Social Security for fear that it would be fruitless, but indicated he might call one more meeting before Dec. 31 in an effort to reach bipartisan agreement on how to save Social Security.

"I assume he thinks there's some hope for some movement," said Democrat Martha Keys, a commission member and former member of Congress.

The commission, created by President Reagan to report by year's end on solutions to Social Security's financing problems, has considerably narrowed the gap between the 10 members appointed by the Republicans and the five named by the Democrats. But it has been unable to agree on any bipartisan plan because Democratic appointees lean more toward raising taxes while Republicans lean more toward cutting benefits.

Members have said they wanted some guidance from the White House and House Democratic leaders on what would be acceptable before locking themselves into a recommendation, and even suggested that the president and House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) meet privately on the issue, but there have been no indications that such a meeting is likely.

Greenspan met with White House chief of staff James A. Baker III for over an hour Wednesday in search of some guidelines that could move the commission closer to agreement, but neither he nor the White House would reveal what was said.

He did tell members of the commission yesterday, however, that he does not favor a divided final report containing a majority Republican plan and a minority Democratic plan. He has said all along that the commission's purpose is to bridge this gap, not polarize it.

In the absence of bipartisan agreement, the commission's final report probably would simply list options for cutting benefits and raising revenues rather than present separate majority and minority recommendations.