President Reagan campaigned today for support from Jews, environmentalists and ethnic blue-collar workers in a three-state northeastern swing that strategists said was designed to boost his strength among traditional Democratic constituencies.

At a synagogue in Valley Stream, N.Y., Reagan denounced racism and bigotry as "unacceptable evils," criticized Democrats for not condemning anti-Semitism at their national convention last July and cited "shared ideals and mutual interests" of the United States and Israel.

The president then flew to Fairfield, Conn., where he signed legislation establishing a four-site wildlife refuge along the southwestern Connecticut coast. He also gave a stump speech, repeated here, that stressed patriotic themes and economic optimism and was given to enthusiastic, flag-waving audiences.

At every stop, Reagan appealed for Democratic votes, reminded audiences that he is a former Democrat and said such Democratic presidents as Harry S Truman "didn't reserve all their indignation for America. They knew the difference between freedom and tyranny, and they stood up for one and damned the other."

Reagan's speech at Temple Hillel was described by Rabbi Morris Friedman, who introduced him, as the first in a U.S. synaogue by a sitting president since George Washington.

Reagan visited a synagogue while campaigning in Southern California in the waning days of the 1980 campaign and also lit menorah candles at the Jewish Community Center in Rockville, Md., last Dec. 4.

Wearing a blue yarmulke inscribed with the presidential seal, Reagan was applauded by the congregation when he declared that church and state must be kept separate. But he added that the government should not make it difficult for believers to practice their faith.

Several listeners interrupted Reagan's speech with shouts of "four more years" but were shushed by other congregation members.

Reagan was given a standing ovation when he repeated a familiar pledge that the United States would walk out of the United Nations with Israel if the Jewish state is expelled from the international organization.

After vowing that there must "never again" be a Holocaust, Reagan drew a relationship between administration policies in Lebanon and preventing such occurrences as the Nazi destruction of 6 million Jews during World War II.

Reagan said it is important that the message of the Holocaust "should again be impressed on those who question why we went on a peace-keeping mission to Lebanon."

White House spokesman Larry Speakes said later that the president believes that U.S. actions in Lebanon were directed at preventing another Holocaust "because the people who want to destroy Lebanon also want to destroy the Jewish state."

On Air Force One, en route to Temple Hillel, Reagan telephoned Mother Teresa, the Roman Catholic nun known for her work with the poor. She had written him Oct. 19 asking food aid for Ethiopia. Speakes said Reagan called her in New York and told her that he would help.

Edward J. Rollins, campaign director of Reagan-Bush '84, said today's trip was aimed at winning Democratic votes and boosting the president's lead in the Northeast, supposedly his weakest region.

However, Reagan polls show that the president has leads of 20 or more percentage points in Connecticut and New Jersey and a 14-point lead in New York.

Campaign press secretary James Lake said Reagan is trying to create "a national mandate" that will pull Republicans into Congress. Even if he fails to do that, Lake said, the president is more likely in 1985 to win cooperation of Democratic congressmen whose districts he has carried.