Opposition from Sen. Jesse Helms (R-N.C.) is threatening passage of the proposed National African American Museum, and backers have been forced to try a potentially risky maneuver to get the facility approved.

The maneuver involves tacking an amendment to the District's annual appropriation -- already a hotly contested issue in the Senate. Because it is the last appropriations bill yet to be passed, and because such bills must be dealt with before the end of September, the D.C. bill has become a legislative pack mule for stalled causes. Yesterday, Helms promised to add his complaints about the museum to the ongoing debate over the District's allocation.

His major concern is the cost of the museum, Helms said in an interview. "Plus the fact that the other groups are going to say, if you are going to do it for them, you have to do it for us."

Helms's opposition masks broad support for the museum in both parties -- yesterday's amendment was jointly filed by Sens. Paul Simon (D-Ill.) and John McCain (R-Ariz.). "The only member of the Senate we have heard any reservations from is Senator Helms," said Mark Rogers, legislative director for the Smithsonian Institution, which is overseeing the project.

But Helms is known as a master of Senate procedure, and by raising more than two dozen questions about the mission and costs of the museum late in the process, he all but blocked the museum authorization as the clock ticks toward next week's scheduled end of the congressional session.

Among the questions Helms asked was: "How will the Smithsonian deal with requests by other groups -- e.g. the Nation of Islam, or other 'black separatist' groups" that might want to use the museum space?

Yet Simon said yesterday that the outlook for the legislation is good once he gets it to the floor. He has lined up a bipartisan roster of 30 cosponsors and 20 additional supporters, and the bill passed the Senate Rules and Administration Committee unanimously earlier this year. The remaining problem, he said, is lack of time.

"It is getting tight and I am getting concerned," said Simon.

If the bill is not approved in this session, a new drive would have to be made next year with a newly elected Congress, postponing the nuts-and-bolts work needed to start the museum.

The museum, which has been approved by the regents of the Smithsonian Institution and came close to full passage by the Congress two years ago, would be the country's largest and most visible museum devoted to the history, art and culture of African Americans. A House version of the legislation was approved last year.

"I think that this is very encouraging. We appreciate the efforts of Senator Simon to move this legislation forward and all that he is doing to make the National African American Museum a reality," said Shireen Dodson, the project's assistant director for administration and planning. "This should not be held up by one person. At least give it the opportunity to go before the full floor and be voted on."

At risk are collections that might go elsewhere or become entangled in court battles if prospective donors die before the museum becomes a reality. Fund-raising opportunities are being delayed and support for the museum might erode if it hovers in a holding pattern too long.

In the last three years, the small staff of five, in preparing for the museum, has made contact with 8,000 people who have vast and rare collections or single items they want to donate. At the beginning, the museum plans to build its collections on gifts, not paid acquisitions.

But each time there is a political delay, even the most enthusiastic potential donors get antsy.

"We have identified a major collection of early-19th-century and 20th-century photographs -- including one important image of Frederick Douglass sitting on a porch at a women's suffrage meeting as the only male," says Deborah Willis, curator of "Imagining Families: Images and Voices," an exhibition of photographs currently showing at the Smithsonian and an example of the direction the proposed museum might take.

This potential donor, Willis said, has "called about eight times this year to see what is happening. ... They want to move on in identifying who would be interested in the collection."

A collector in England has photographs of Africans aboard a slave ship as late as the 1870s; families across the country have been looking for places to send their loved ones' manumission papers or their relatives' Pullman porter and Negro League baseball uniforms.

These gifts can't be accepted until there is full authorization from Congress. "A lot of people know about the photographs in England because we published one of the photos in our newsletter. People are interested," Dodson said. But if the bill does not pass, "someone will end up making {the donor} an offer which we cannot begin to match and then the collection will be lost to the museum and maybe to African Americans in this country.

"And our oral history is in jeopardy," she continued. "People are dying; one woman we interviewed last year is now senile."

But Helms said yesterday he intends to fight to the end. "It will be contested," he said of the District appropriation and the new museum amendment. "I have 15 amendments that I'm going to add, so we might not get to vote on it until 1996."