President Reagan yesterday rejected a proposal to delay military aid to the Nicaraguan rebels for six months to allow time to negotiate with the Sandinista government.

But he hinted that he would be willing to seek some type of compromise on his proposed $100 million aid package for the counterrevolutionaries, or contras, if it is defeated in a House vote March 19.

The president is waging an uphill lobbying campaign to push his $100 million package, which includes $70 million for arms, through the House. "The alternative is unthinkable," he said yesterday after returning from Camp David.

But he is facing strong opposition from Democrats. Rep. Richard A. Gephardt (D-Mo.) said yesterday on ABC News' "This Week With David Brinkley" that many fear that Reagan's intent in seeking weapons for the contras is "to simply get rid of the Sandinistas," not just to pressure them to broaden their government, remove Soviet and Cuban advisers and refrain from financing communist guerrilla movements.

Gephardt, as well as Sen. Christopher J. Dodd (D-Conn.) on NBC's "Meet the Press," urged the president to seek a solution through negotiations and diplomatic efforts by other Latin American nations before giving arms to the contras.

But the president yesterday turned down the proposal of Sen. Jim Sasser (D-Tenn.) that any military aid to the contras be held in abeyance for six months to let negotiations go forward. "I think that would be asking too much. I think it would be counterproductive," Reagan said, according to wire service reports.

With a short time left to gather votes in the House, the president is scheduled to meet with members every day this week. There were suggestions yesterday that the White House is moderating its earlier attacks on Democrats. Defense Secretary Casper W. Weinberger said on the ABC program that "the president has never questioned anybody's motives" in opposing the aid.

Assistant Secretary of State Elliott Abrams said on "Meet the Press" that Secretary George P. Shultz, meeting with members of Congress earlier to win support for the package, had said, "Every man in this room is a patriot." Abrams denied that the administration wants to overthrow the Sandinistas.

The president said discussions on aid alternatives would "not be productive at all" before the vote, but hinted that he might be open to compromise later, saying the administration is "willing to talk" with Congress about "how we can get practical aid" to the rebels to "pressure the Sandinistas into negotiating for a democratic peace."

He declined to discuss the shape of a possible compromise, saying, "I would have to wait to see what someone offers."