Three liberal Democratic Harvard Law School professors yesterday endorsed Republican Governor Francis W. Sargent in his campaign against the Democratic challenger Michael Dukakis.

At an afternoon press conference in Boston, Professors Stephen G. Breyer, Charles Fried and Charles R. Nesson '60 called upon the voters of Massachusetts to return a liberal of demonstrated honesty and courage to office, rather than a man who has not yet proven himself.

Honest Liberal

"Sargent has honest liberal instincts," said Nesson, former defense counsel for Daniel Ellsberg '52, and Daniel and Philip Berrigan. "Those liberals who support Dukakis are making the kid's mistake of trading a bird in the hand for two in the bush. In the process, they are doing a disservice to bipartisan liberalism," he said.

Three law professors said that a Sargent defeat would show that politicians of principle and integrity cannot be reelected.

Eroding Respect

Although the three professors did not sign a statement expressing disappointment with Dukakis that was released at the press conference, all of them said yesterday that their respect for Dukakis has eroded during the campaign.

"Sargent has done the courageous think even when it was bound to cost him, but Dukakis has deliberately and willfully passed by those opportunities for courage which he has had," Fried said.

Martin Peretz, master of South House and lecturer on Social Studies, said last night he was "glad" the three professors agreed with him in supporting Sargent. Peretz, also a liberal democrat, announced his endorsement of Sargent in August and reportedly has contributed money to the incumbent governor's campaign.

"The three are shrewd and idealistic men," Peretz said.

Three other liberal Democrats, Martin Levin, director of the urban affairs program at Brandeis University, William P. Homans, Jr. '41, a Boston criminal lawyer, and Patrick Jones, director of the Lena Park Community Center, joined the three law professors in endorsing Sargent.