The Democratic National Committee has launched its latest fund-raising ploy with a letter from two veteran congressional Democrats appealing for help in resisting Sen. Jesse Helms (R-N.C.) and the forces of the New Right the Democrats claim "now control the Republican Party."

The letter, signed by Sen. Daniel Patrick Moynihan (N.Y.) and Rep. Morris K. Udall (Ariz.), is the first mailing for the financially beleagured DNC by the direct-mail firm of Craver, Mathews, Smith. The initial distribution is a modest 60,000 letters -- all seeking money from past contributors to the DNC.

Robert Neuman, a spokesman for the DNC, said Moynihan and Udall were chosen as co-signers because "we wanted to draw from all elements of the party and attract both moderates and traditional liberals."

The letter prepared by the direct-mail firm is marked by the absence of any direct attack on President Reagan, whose name appears nowhere in its four pages. Addressing people who have given in the past but not this year, it says, "Perhaps you have not already renewed your support . . . because you are disillusioned. . . . You might feel that the Democratic Party is hopelessly torn between rival factions and that no new leader has come forth to carry the banner. . . .

"Last November," it says, "the American people decided to turn the Democrats out. They voted for change.It was their intent to bring new ideas and new leadership to Washington. We accept that."

But the letter contends that the mandate of November has been distorted into a demand for such things as repeal of the Voting Rights Act, outlawing all abortions, turning away older people from Social Security, crippling day-care centers, giving developers a free hand on public lands, and selling sophisticated weapons to Saudi-Arabia.

All this, it says, is because such New Right leaders as Helms have taken control of the Republican Party and are setting its goals.

Neuman said copy for the letter was prepared before Reagan nominated Judge Sandra D. O'Connor of Arizona for the Supreme Court, incurring the wrath of many of the New Right leaders who object to her reported views on abortion. Ironically, Udall has written newspaper articles praising Reagan for picking O'Connor in the face of the New Right criticism.

Neuman said the direct-mail firm is preparing a series of "segmented" appeals, signed by other Democrats, to particular constituencies in prospecting for new sources of contributions. The Democrats have lagged far behind the Republicans in the use of direct-mail solicitation, which has become a major source of funds for the GOP.