WASHINGTON, Oct. 30 - With the announcement of a new Supreme Court nominee expected as early as Monday, Senator Harry Reid of Nevada, the Democratic leader, warned President Bush on Sunday not to pick one of the candidates said to be on the president's short list, Judge Samuel A. Alito Jr.

"I think it would create a lot of problems," Mr. Reid said on "Late Edition" on CNN.

Republicans close to the selection process said over the weekend that as Mr. Bush neared a final decision, Judge Alito, of the United States Court of Appeals for the Third Circuit, remained a leading candidate, along with Judge J. Michael Luttig of the Fourth Circuit, Judge Alice M. Batchelder of the Sixth Circuit, and Judge Priscilla R. Owen of the Fifth Circuit.

One outside adviser to the White House said on Sunday that a surprise was still possible. But prominent conservatives said all four have the kind of clear record and judicial credentials they are hoping for in a Supreme Court nominee, and liberal groups are girding for battle to oppose them.

Mr. Reid had already said he would object to the selection of Judge Luttig or Judge Owen. And on Sunday, he did not rule out the possibility that Democrats would try to block a nominee by a filibuster or refusing to close debate and vote. "We are going to do everything we can" to see that the president names "somebody that's really good," Mr. Reid said.