The Senate minority leader, Tom Daschle of South Dakota, said today he thought there was enough support to sustain the veto, but added that it was "a very close call," which could go against the Administration if conditions in the Balkans deteriorated further.

The Senate majority leader, Bob Dole of Kansas, agreed about the volatility of the situation. "Things can change very quickly in that part of the world," he said. But Mr. Dole, who supports the legislation, said Mr. Clinton "has not suggested anything but minor modifications to a failed policy."

The bill would require the President to end American support for the embargo after the withdrawal of United Nations peacekeeping forces now there, or within 12 weeks of a request to do so by the Muslim-led Bosnian Government, whichever came first.

The veto came as Mr. Clinton's national security adviser, Anthony Lake, and other senior officials were traveling in Europe, seeking support for a peace plan that would establish ethnic zones in the former Yugoslavia, and added stops for discussions with officials in Russia, Italy and Spain. In a news conference on Thursday, Mr. Clinton said, "This could be a moment of real promise."

In an interview taped with MTV Music Television News this morning, Mr. Clinton said that the recent drive by Croats meant "the Serbs have been rolled back in the western part of Bosnia." He added, "I hope we have a chance to make a decent peace there." He repeated his longstanding view that he would not oppose all the parties to the embargo lifting it in the event the United Nations mission failed and withdrew.