The Palestinian bid for full membership in Unesco — the United Nations Educational, Scientific and Cultural Organization — has put both Washington and the organization into an urgent bind.

United States legislation dating back more than 15 years mandates a complete cutoff of American financing to any United Nations agency that accepts the Palestinians as a full member. Unesco depends on the United States for 22 percent of its budget, about $70 million a year.

Neither the Obama administration nor Unesco wants the cutoff to happen, and diplomats are desperately negotiating with Congress, the Palestinians and other Unesco member states to find a resolution that will preserve the agency’s budget. But with a vote on membership coming as early as this week, time is running out.

Unesco, perhaps most famous for designating world heritage sites, is a major global development agency whose missions include promoting literacy, science, clean water and education, including sex education and equal treatment for girls and young women. To some degree, a senior American official said, Unesco helps promote Western values under an international umbrella in places where an American one might be resented or misunderstood.

That is one reason, the official said, that the United States rejoined the organization under President George W. Bush after the Sept. 11, 2001, attacks.

Unesco membership “is in the core security interests of the United States,” the agency’s director general, Irina Bokova, said in an interview here. “I think the United States should take a very careful look at this legislation, in their own interests. I don’t believe it’s in the U.S. interest to disengage from the U.N. system as a whole.”

The irony is that the Obama administration agrees and has been a strong supporter of Ms. Bokova. But lawyers at the State Department see no way around the laws, which date from 1990 and 1994 and provide no possibility of a presidential waiver.

American officials have criticized the Palestinian move, part of its bid for full membership in the United Nations, as “premature.” They fear it will lead to more conflict with Israel and further undermine the possibility of peace talks.

Despite American objections, Unesco’s 58-nation executive board approved the Palestinian application this month. The agency’s general assembly meets here starting Tuesday; the 193 member countries are scheduled to vote on Palestinian membership during the two-week meeting and are expected to approve it.

Unesco has long been viewed in the West as politicized, corrupt and anti-American, antipathy that came to a head in 1984 when President Ronald Reagan withdrew American membership. Since then, the agency has been much reformed, both in terms of its finances and its embrace of values like freedom of the press and education for women.

Hillary Rodham Clinton was the first American secretary of state to visit Unesco, coming this year to support an initiative on education for girls and young women, and Ms. Bokova emphasizes that since 9/11, Unesco has run its largest education project in Afghanistan, opening literacy centers for civilians as well as Afghan police officers. It cooperates on teacher training with American companies like Microsoft, she said, and has organized training for Tunisian and Egyptian journalists since the Arab Spring revolts.

Only this month, the United States made separate voluntary contributions to Unesco programs for education and clean water; Washington praises its work on behalf of universal literacy, gender equality and disaster preparedness.

If the United States withdrew its financing, it would still retain a seat at the agency for another two years, but even then its influence would be weakened.

“In a world where soft power is so important, the United States is counterproductively compromising its position in a forum that really matters,” said Ronald Koven, who monitors Unesco for the World Press Freedom Committee, an American nongovernmental organization.

Peter Yeo, vice president for public policy at the United Nations Foundation, which supports the organization’s goals, said that “what’s maddening is that this is not your grandfather’s Unesco — it is better managed, more efficient and U.S. leadership in Unesco has made it a better organization.”

Mrs. Clinton has asked the American special envoy to the Middle East, David M. Hale, to negotiate with the Palestinians and Arab countries to break the impasse. The State Department has said it hoped to press the Palestinians to withdraw their request.

There have been discussions about inviting the Palestinians, longtime nonstate observers at Unesco, to sign three major conventions — including the World Heritage Convention, which could list key sites currently under Israeli control as Palestinian — as a nonstate signatory, the way the European Union has done. Such a move would give the Palestinians some of the advantages they seek in joining Unesco without full membership.

Repeated requests to interview the Palestinian ambassador to Unesco or his deputy were declined. Palestinian officials have previously said they see membership as part of the recognition they seek as a state, which the Palestinian envoy to Unesco, Elias Wadih Sanbar, referred to as “a new era in which Palestine is recognized.”

An Arab ambassador, speaking on the condition of anonymity, said there was also discussion of approving full membership for the Palestinians but delaying it for six months, although that would not prevent a cutoff of American money. There is also talk that other Arab states could make up the shortfall in the Unesco budget.

But Arab representatives say that it will be very difficult for Mahmoud Abbas, the Palestinian president, to compromise. Any “package deal” short of membership, one of them said, “would look like bribery.”

Unesco has a two-year budget of $643 million for 2010-11 and a projected budget of $653 million for 2012-13. Since the United States normally pays its 22 percent share toward the end of the year, a cutoff could mean no payment for 2011, another $70 million blow to the budget. The result would be immediate cuts in programs and personnel.

Ms. Bokova is hopeful for a resolution, but she said that on a recent visit to Washington she found “skepticism and lack of knowledge” about today’s Unesco.

Likewise, American officials doubt Congress will alter the legislation. Many Republicans, who control the House, are hostile to both the United Nations and the Palestinian statehood bid.

“We have a very strong case to make,” Ms. Bokova said. “Unesco is very different from 25 years ago.”