WASHINGTON (AP) _ A leading Democratic supporter of aid to the Nicaraguan Contras says the policy is being undercut because the Reagan administration's point man on the issue has lost credibility with Congress.

Sen. Sam Nunn, D-Ga., says that while he endorses ''the general policy that Elliot Abrams has been articulating ... his credibility has been severely damaged.''

Abrams, an assistant secretary of state, came under renewed fire Sunday for misleading lawmakers about administration efforts to help the Nicaraguan guerrillas during a period when Congress had banned such aid.

President Reagan and Secretary of State George Shultz have said Abrams will remain in office. But Nunn, chairman of the Senate Armed Services Committee and a member of the panel investigating the Iran-Contra affair, said Abrams' involvement in further discussions of the issue will hurt the prospects for Contra aid.

''When we debate the overall aid to Contras in the fall of this year, his presence will not be helpful,'' Nunn said on the NBC-TV program ''Meet the Press.''

The administration reportedly plans to ask Congress to approve a $150 million package to send military and other aid to the Contra over 18 months. But the chances of congressional passage have been dimmed by the furor over White House use of proceeds from secret arms sales to Iran to help fund the Nicaraguan guerrillas.

Reagan's own job security came into question Sunday on ABC's ''This Week With David Brinkley.'' Rep. Lee Hamilton, D-Ind., the chairman of the House committee investigating the Iran-Contra affair, was questioned about a memo written by former National Security Council aide Lt. Col. Oliver North regarding a proposal to divert Iran arms sales profits to the Contras.

The memo, drafted by North in April 1986, has not been linked publicly to Reagan, and the president has denied all knowledge of the diversion of funds.

Hamilton said he had no information that would link Reagan to the memo, but when pushed to speculate on what might happen if such a linkage were discovered, he said, ''I think it is likely, if that occurred - and let us emphasize the 'if' - that if it occurred, you would have a demand for impeachment proceedings.''

Last year, Congress voted the Contras $100 million in aid, ending a nearly two-year period in which the administration was barred by law from giving the guerrillas military assistance.

Administration officials used a variety of means to circumvent the ban on military aid, including a request from Abrams to the sultan of Brunei for $10 million to be deposited in a Swiss bank account. Someone along the line got the account number wrong, however, and the money never reached the Contras.

Abrams, testifying before the Iran-Contra committee, said he had not disclosed the Brunei affair during earlier classified testimony before the Senate Intelligence Committee because he had not been authorized to do so by Shultz and because he had promised Brunei officials that the matter would remain secret.

Abrams was defended Sunday by Rep. Dick Cheney, R-Wyo., a strong backer of Reagan administration efforts to win more congressional aid for the Contras.

''My feeling is that Elliott should not be thrown over the side,'' Cheney said on the Cable News Network program ''Newsmaker Sunday.''

''Hopefully, he has learned a lesson from this experience,'' Cheney said.

The State Department, meanwhile, is warning that Iran will be held responsible for the safety of American hostages if they have been taken there from Lebanon.

Ash-Shiraa, a weekly magazine in Beirut that last year broke the story of secret U.S. arms sales to Iran, is now reporting that some of the eight Americans held hostage in Lebanon have been taken to Iran. It says one faction of the government wants to put them on trial.

The Iranians denied the report. ''The Iranian Islamic Republic has nothing to do with the hostages,'' said a spokesman at the Iranian embassy in Beirut.

''We have no information which would substantiate these reports,'' said State Department spokesman Pete Martinez. ''However, if such reports were indeed true, we would consider it a matter of the utmost gravity and would hold the Iranian government directly responsible for the safety and well-being of the hostages.

''The very notion of a 'trial' for the hostages is outrageous. The hostages are not criminals but innocent victims.''