Poor Jimmy Carter. With friends like his, he hardly needs enemies. Now secret State Department documents and reliable intelligence sources reveal that the latest volunteer to board the President's bandwagon is none other than Cuban President Fidel Castro, who has endorsed Carter for reelection.

So far, to the intense relief of Carter's campaign staff, the Marxist dictator has refrained from a strong public endorsement. The closet he has come was a blast at the Republican platform.

But in private conversations with American and Nicaraguan officials last month, Castro expressed support for Carter that was as positive as it was unsolicited. He indicated that he'll do nothing that would put Carter on the spot before the November election.

One conversation was between Castro and Carter's United Nations ambassador, Donald McHenry, at an informal reception July 18 in a former Managua country club. Delegations from 30 nations had arrrived in the Nicaraguan capital for ceremonies the next day marking th first anniversay of the successful Sandinista revolution.

One of those present at the recption said the 45-minute Castro-McHenry conversation was not planned -- or at least appeared not to be. "People were mingling, and Castro and McHenry were at different sides of the room before they found themselves together," the source said.

At any rate, according to intelligence sources, Castro pledged his firm support for Carter's reelection bid -- clearly intending his views to be relayed to the White House by McHenry. Secret cables to Washinton reported Castro would do nothing to embarrass Carter before Election Day on Nov. 4.

In other conversations, some with Nicaraguans and one involving U.S. Ambassador Lawrence Pezzullo, Castro was even more explicit. A highranking intelligence source told my associate Dale Atta that Castro "passed along a virtual promise that he would cause no crisis during the next four months."

The source added: We have taken that to mean the military improvements, and the heightened activity by the Russians in Cuba, would become less visible, or would stop."

As I have reported in recent months, Castro and the Soviets have been beefing up the Cubans' military strength at an alarming rate in the past year, particularly in the missile department.

It's evident that Castro, remembering Carter's pathetic backdown on the Soviet combat brigade last year, views him as a president who can be pushed around.

A Castro promise not to embarrass Carter is of course an embarrassment in itself. As one White House source noted: "No presidential candidate would ever want to receive Castro's public support.