In one of the first clear cracks in Republican Party unity since Ronald Reagan's inauguration, the new chairman of the GOP's Ripon Society put the president on notice yesterday that party moderates are no longer willing to march in lock step with his administration.

Declaring "Reagan must be saved from Reaganism," Rep. Jim Leach (R-Iowa) issued a "Moderate Manifesto" challenging many of the tenets of the Reagan foreign and domestic policies.

Leach said he was speaking officially in behalf of the society, long the home and refuge of the party's middle-road minority.

He charged "negativity" is becoming the "driving force" behind the administration, that the United States "erred profoundly by sending military advisers to El Salvador," that Reagan's tax bill was "overly generous" to the rich and that the administration has put "a dangerously exclusive emphasis" on building up arms while ignoring arms control.

On the domestic front, Leach said the administration "is insensitive to our constitutional heritage" by refusing to commit itself to women's rights, fair housing and affirmative action. As for the forthcoming round of further budget cuts, he said they must concentrate on the military budget, public works programs, subsidies to the tobacco, sugar and peanut industries, and assistance to the Clinch River breeder reactor.

Leach insisted the manifesto was not meant as an attack on Reagan. But it was a clear attempt to set an alternative agenda for party moderates, and give voice to fears that conservatives are dominating administration policy.

Leach said moderates generally support the thrust of the president's programs, but are becoming increasing concerned about the "narrow and compassionless" way policies are being implemented.

"We are concerned that on a series of fundamental issues the ideologues on the right will prevail over the principles many of us consider as representative of historic Republicanism," he said.

The manifesto marked the most comprehensive critique of the Reagan administration to date from the GOP's moderate wing. Its author is a second-term congressman from southeastern Iowa, and a member of the "gypsy moths," an informal caucus of northeastern and midwestern Republicans. He was an early supporter of George Bush in the 1980 presidential race.

Although the manifesto was not endorsed by the group, Leach said many of the "gypsy moths" support his views.

In it, he proposed a one-year halt on spending on the MX missile system, reopening of the strategic arms limitation talks, reinstatement of the windfall profits tax on the oil industry to bolster Social Security trust funds, linkage of arms sales and human rights records, a crackdown on antitrust violators, and moving at least one Cabinet department from Washington.

He also called the policies of Interior Secretary James G. Watt "an aberration" from GOP traditions, and suggested Watt be named ambassador to Austria.

Some of Leach's harshest words were directed against Reagan foreign policy. The administration, he said, has an "almost exclusive paranoia against the Soviet Union with no complementary concern for the compassionate needs of the less advantaged peoples of the world."

Reagan has let "extremism" dominate his human rights policy, and is missing an opportunity to "ensure a generation of Republican government" by failing to press for strategic arms limitation, he said.

Leach also expressed fear that the Republican Party is becoming "lashed to the guillotine of the New Right," and its "politics of polarization."