A coalition of religious leaders seeking federal backing for an ambitious plan to reopen a Youngstown mill yesterday won the promise of a $100 million federal loan guarantee if it comes up with a plan acceptable to the government.

In addition, presidential assistant Jack H. Watson Jr. told a delegation from the Mahoning Valley (Ohio) Ecumenical Coalition that their proposal for reopening the plant under community ownership will "get priority attention" from the administration's lateragency Coordinating Committee, of which he is chairman.

Watson who chaired the 95-minute White House meeting with the coalition leaders, promised to get back to them within three weeks with the federal government response to the proposal.

Yesterday's promise of federal assistance came a little over a year after the colsing of most of the Campbell Works of the Youngstown Sheet and Tube Co., which ultimately cost the community an estimated 7,700 jobs.

Even before the plant closed, Protestant, Catholic and Jewish leaders had come together, first to deal with the shock of the sudden, massive unemployment and then to see if economic alternatives could be found.

The Ecumenical Coalition commissioned a $350,000 study, funded by the Housing and Urban Development Department, by the Washington-based National Center for Economic Alternatives.

The study concluded that it was technically feasible to reopen the plant as a worker-and-community-owned operation. But the cost for the unique venture was put at $525 million over a period of eight years, with most of the money coming from loans.

Robert T. Hall, assistant secretary of commerce, who also met withthe coalition leaders yesterday, said that the $100 million in guaranteed loans has been "set aside for a project which is economically viable" to deal with the Youngstown situation.

The report by the National Center for Economic Alternatives envisions loan guarantees of $300 million, a figure Watson told reporters he did not consider outlandish. He added that he had "very little doubt that we can, between the public sector and the private sector, put something together that cna work."

The coalition will also make a formal request for a $16 million grant HUD, according to the Rev. John Sharick, vice chairman. He said this would be part of an estimated $50 million needed to purchase the cosed down plant and put together a management team to operate the facility.

Within the past eight months the coalition has raised a little more than $4 million from Youngstown local residents, and organizations.

Some of the church leaders indicated privately that they were counting on the political pressures of the November election to give them added leverage with the Carter administration.

A poll commissioned by the Republican National Committee and published Sept. 10 by the Youngstown Vindicator showed that both President Carter and Democratic incumbent Congressman Charles J. Carney trailing markedly in the perception of voters as to who is "having an impact" on the area's economic crisis.

"This political thing works in our favor," observed the Rev. Charles Rawlings of Cleveland.

"I don't think the Democrats can win without our support," he said of the large numbers of clergy throughout Ohio's industrial area allied with the Coalition. That support, he indicated, would be for many clergy, contingent on the administration's reaction to the economic crisis.