The Democratic Party still is trying to make political hay and dollars out of the Social Security issue, despite the supposedly bipartisan rescue plan agreed to by President Reagan and House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.).

Democratic National Committee Chairman Charles T. Manatt, in a fund-raising letter, says the party urgently needs $500,000 to counter a lobbying campaign, "led by the ultra-conservative New Right," to cut Social Security benefits.

The letter says the Social Security commission report is "an encouraging first step," but it notes that Sen. William L. Armstrong (R-Colo.), chairman of the Social Security subcommittee, opposed the bipartisan report because "it didn't cut benefits enough."

It warns that "the critical vote in Congress is now at hand" and that the fate of Social Security shouldn't be left "in the hands of those who remain locked into harsh benefits cuts."

"Message from November elections is clear: Americans will not stand for drastic cuts in Social Security," Manatt adds in telegram-like prose. "Urgent that we not let conservative special interests undermine that message in Congress."

The letter, part of a campaign to build a list of donors to the party, was sent to 3 million households on Monday. It is the second letter on this issue signed by Manatt. The first, mailed before Christmas, raised about $450,000.

Asked to comment on the letter, Bill Greener, communications director for the Republican National Committee, said, "It's interesting to note [it] makes specific and complimentary reference to the bipartisan proposal, made under President Reagan's leadership."