THE UNITED NATIONS is caught up in an immense financial crisis, and two reactions to it are evident. The first is a yawn and the second is a smirk. Institutional crises at the United Nations do that to a lot of Americans. But it is precisely in this space between apathy on one side and antagonism on the other that damage to the American interest could be done.

For years the United Nations has limped along under the burden of slow or no payment of assessments (dues) by various members. What turns that limping into buckling is mainly the new American withholding of some $100 million. Three acts of Congress are responsible. The Kassebaum Amendment, intended to reduce the American share of the budget from 25 percent to 20 percent, cuts $42 million. The Sundquist Amendment withholds the American share ($20 million) of the cost of salaries of secretariat staffers (mostly from the Soviet bloc) who are said to fork over part of their pay to their governments. Gramm-Rudman-Hollings does the rest.

The U.N. has long needed a budgetary keelhauling. Secretary General Javier Perez de Cuellar is making a dogged effort. But he can only trim, to a point, the costs of authorized programs. It falls to the members to go on, after they arrange stopgap financing, to trim the programs, necessarily a political exercise. The big contributors, Moscow as well as Washington, share a desire for budget reform, and together are applying a financial squeeze to bring it about. They differ, between themselves and each with the Third World, on program reform. (How silly the United States looks, by the way, complaining of the U.N.'s spendthrift ways while it provides rent subsidies of up to $10,000 a month to members of its mission in New York.)

The way the United States is bringing pressure, meanwhile, remains troubling. The unilateral withholding of assessments tramples on the U.N. Charter, which makes paying them a formal treaty obligation. The terms of a treaty commitment bearing on assessments can be renegotiated. It is wrong simply to renege. It apes the old Soviet (and French) practice, which the United States roundly and rightly condemned.

The Reagan administration came in expressing a generalized hostility to the U.N. and has been slow to adapt its policy to its actual mixed experience there. By its own tough advocacy and by the coincidence of the Afghanistan issue, it has found the U.N.'s political forums more rewarding than its ideologues expected. All this is apart from the U.N.'s specialized agencies, of which many do important work and all will be affected by cuts in American voluntary contributions. The Reagan administration wants to be known as committed to internationalism, but it still rebels against the familiar frustrations of dealing in the world organization. It wants less expense and more influence. It has to figure out which it wants more.