If Richard C. Holbrooke thought he had leapt the last barrier to his confirmation as U.S. ambassador to the United Nations, he'll have to think again.

The diplomat-cum-investment banker emerged from three days of hearings before the Senate Foreign Relations Committee on Thursday apparently assured of approval, a year after President Clinton selected him for the job.

But not so fast. Sen. Charles E. Grassley (R-Iowa) has exercised his prerogative to put a hold on the nomination -- not because he opposes Holbrooke, but because he wants the State Department to rescind its decision to discipline Linda Shenwick, a senior civil service employee at the U.S. Mission to the United Nations in New York.

By tradition, any senator can hold up -- at least for a time -- the confirmation of a judge, ambassador or other official. The nominees become bargaining chips in whatever dispute the senator has with the administration.

A week ago, Shenwick was booted from her New York office and ordered to transfer to Washington to take up what her lawyer, Victoria Toensing, called "a made-up job ordering furniture." Toensing said the transfer was "a personal vendetta" by Secretary of State Madeleine K. Albright, for whom Shenwick worked when Albright held the U.N. post during Clinton's first term.

State Department spokesman James P. Rubin responded that Albright "treats all her employees fairly and according to the rules and has no personal vendettas."

In a statement Thursday night announcing the hold, Grassley portrayed Shenwick as a whistleblower who has revealed waste and mismanagement at the U.N. "Some people in the State Department, apparently all the way to the top, don't much care for Ms. Shenwick's candor with Congress. And so they painted a big ol' target on Ms. Shenwick and have come after her relentlessly," he said.

The Shenwick case is not new. Three years ago, seven Republican senators held up the nomination of career diplomat Princeton Lyman as assistant secretary of state for international organizations over Albright's decision not to renominate Shenwick to an obscure U.N. budget committee.

Grassley accused the State Department of reneging on an agreement to settle the earlier dispute. "That will not happen again," he said. "The hold gets lifted when there's an agreement in writing."

CAPTION: Richard C. Holbrooke's confirmation had been considered assured.