The Democratic Party, which battered the Republicans on Social Security during the 1982 congressional election campaign, last month sent out a new fund-raising letter charging that Republicans "have always fought Social Security" since passage of the act in 1935 and "in the past 47 years, little has changed."

The letter, sent after Christmas to 3 million people by the Democratic National Committee, became an issue yesterday as leaders of the President's Advisory Commission on Social Security continued closed-door negotiations seeking a bipartisan plan to save the system, which is running short of money to pay old-age benefits.

No agreement was reached, and another meeting reportedly was scheduled for today.

One of those involved, Senate Finance Committee Chairman Robert J. Dole (R-Kan.), reportedly told associates that "partisan potshots" like the DNC letter were "not helpful" and could undermine the negotiations.

However, Bob Neuman, spokesman for the Democratic National Committee, said, "I don't think the tone of the letter undermines efforts to reach a compromise."

Neuman said the letter was getting a "good response," bringing in about $60,000 a day.

Overall, he said, however, the expected proceeds of $400,000 to $500,000 would not be much higher than its mailing costs.

Signed by DNC Chairman Charles T. Manatt, the letter said:

"It is simply too dangerous to leave the fate of Social Security in the hands of the Republican special interests who really don't support it" and who are "intent on reducing record-high budget deficits regardless of the cost in human hardship and suffering."

"The Republicans have always fought Social Security," the letter said.

"When Franklin Delano Roosevelt and the Democrats enacted the program in 1935, the biggest opponents were Republicans. In the past 47 years, little has changed," it added.

This language referred to the fact that in 1935 a majority of Republicans in both chambers of Congress voted for motions to kill the old-age insurance provisions.

Since then, contrary to the implications of Manatt's language, there has never been a majority in either party voting to kill the program.

The letter concluded with an appeal for money to "explain the truth about Social Security and dispel the distortions being put forth by Republicans."