The National Council of Churches, the country's largest interfaith organization, is under the sharpest attack in its history for an alleged leftward tilt in its politics.

Led by the recently formed Institute on Religion and Democracy, a conservative group, critics have accused the council of financing humanitarian programs that, whether or not the council realizes it, support Marxist-Leninist governments, revolutionary guerrillas and anti-American organizations.

Their charges were picked up in a recent article in Readers' Digest and in a broadcast Sunday night on CBS-TV's "60 Minutes" program, and have brought a sharp new immediacy to an age-old quandary: how can the church be involved in the cause of social justice without becoming politicized?

"We are not criticizing the church because of social involvement, what we are concerned about is a bias toward the totalitarian left," the Rev. Edmund Robb, chairman of the executive committee of the institute, said in a news conference here yesterday.

Specifically, the institute has faulted the National Council for:

* Giving nearly $500,000 in humanitarian aid to Vietnam's "New Economic Zones," which critics say are nothing more than concentration camps for political undesirables.

* Providing $15,000 to EPICA, the Ecumenical Program for Interamerican Communication and Action, a group that has crusaded against what it sees as U.S. colonialism in Puerto Rico.

* Producing a film strip on the war in El Salvador that critics say is blatantly sympathetic to the communist guerrillas. At one point the film narrator says: "Many of the El Salvadoran guerrillas have been branded communists, but everywhere I walked I saw the cross of Christ . . . . For our western minds and hearts, to see this juxtaposition of the cross and gun is a shock. But in our own history through the Revolutionary and Civil Wars, we have frequently sought God's help in fighting the forces of injustice."

* Issuing a series of anti-American and pro-communist statements. While he was president of the national council from 1979 to 1981, for example, the Rev. M. William Howard embarked on visits to those he described as U.S. "political prisoners."

And after disillusioned antiwar activists protested human rights violations by Vietnam, James Armstrong, then a Methodist minister and now president of the national council, joined in a newspaper ad that said, "The present government of Vietnam should be hailed for its moderation and for its extraordinary effort to achieve reconciliation among all of its people."

The leadership of the national council, a 23-year-old organization that consists of 32 mainline Protestant and Orthodox groups representing about 40 million Christians, is accustomed to attacks from the political center and right. It came under fire in the 1960s for being in the forefront of the civil rights and antiwar movements, and its publication of a revised standard version of the Bible in the 1950s was anathema to many fundamentalists.

"The NCC has always stood above all political systems," said the Rev. Paul McCleary, executive director of the Church World Service, the NCC's main humanitarian arm, which funds relief activities in about 90 nations.

"Our interest is in helping those who are in need and in furthering justice . . . . We do not fund or support communism. In countries with communist governments, like Vietnam or Cambodia, we determine how we can help people who are suffering only by providing specific material for specific purposes."

The institute's Robb said reaction to the "60 Minutes" broadcast has been overwhelming. "We have had calls from parishes all over the country that want to put their money in escrow," he said.

Of the NCC's $44 million annual budget, roughly a third comes from Sunday collections in the churches. However, according to Harriet Ziegler of the national council, less than one-tenth of 1 percent of the money collected each week in churches goes to the National Council of Churches. The bulk of the money stays local.

Robb's group has called for Christians who are upset with the way their money is spent to remain in their denominations. "We are supportive of the National Council of Churches, and we want to reform from within," he said. "We don't want out, we want in."

Pastor Richard John Neuhaus, a consultant to the institute, which is funded mainly by contributions from conservative foundations, said yesterday, "I think there is an awareness [among the National Council of Church's leadership] of a deep and broad problem, but it will take an extraordinary act of church leadership for the problem to be corrected."