Chief Justice William H. Rehnquist, who will rule over President Clinton's Senate trial and cast tie-breaking votes, has long feared Congress' unchecked power to impeach a president.

The chief justice is known to have been privately surprised and disappointed when the House of Representatives actually impeached Mr. Clinton on Dec. 19, the first time an elected U.S. president was so disgraced.

"It just really significantly impairs - if it doesn't cripple - a president to be the subject of an impeachment trial," Chief Justice Rehnquist, 74, said in a 1992 interview about his long involvement with the theory and practice of impeachments, which includes writing the Supreme Court decision that affirms the very congressional power he opposes.

These days the impeccably impartial jurist won't let the word "impeachment" pass his lips as he prepares to wield the gavel at the nation's second presidential trial.

But for at least 35 years the chief justice has warned that removing a president by impeachment would forever blur the Constitution's separation of powers, leaving Congress supreme instead of one of three equal branches of government that the framers intended.

"Had the Senate, which tried and acquitted Andrew Johnson in 1868, chosen instead to convict him, a long shadow would have been cast over the independence of the president," he said in a 1990 speech at Northwestern University's law school.

While objecting on principle to the constitutional impeachment power over presidents, the chief justice has been less clear in defining an impeachable offense, which he may be called upon to do in Mr. Clinton's Senate trial when deciding on motions to dismiss some of the charges.

Chief Justice Rehnquist considers perjury an impeachable offense, but in his 1997 opinion allowing a trial to proceed immediately in the Paula Jones case he said impeachable offenses must involve "official acts." The charges of sexual misconduct involved in that case are covered by civil and criminal law.

He has not been called upon to decide if either perjury to a grand jury or obstructing the trial his decision authorized - the basis of the two articles of impeachment approved by the House last month - is a "purely private act."

Despite his apprehensions about the Constitution's impeachment clauses, however, Chief Justice Rehnquist's decision specifically permits the controversial Senate trial rules now in force.

As author of the 1993 Supreme Court opinion giving the Senate virtual free rein in trying impeachments, he upheld rules that permit trial by committee and votes on guilt or innocence by senators who may not have heard any evidence. He ruled that federal courts should not interfere in what is "strictly a political question."

Six years ago, when the prospect of presiding over a presidential impeachment seemed to him remote, the chief justice discussed the subject with C-SPAN's Brian Lamb for the program "Booknotes."

"Well, I wrote the book," he boasted when asked how he recalled so much impeachment minutiae from "Grand Inquests: The Historic Impeachments of Justice Samuel Chase and President Andrew Johnson."

But the chief justice was less forthcoming three months ago when Mr. Lamb asked him if he was philosophically opposed to impeaching a president except for "the most serious crimes against the nation."

"That's a question I don't want to answer," Chief Justice Rehnquist said, saying it matters little how the public takes that reply. "Just let them feel I'm difficult."

As the nation's most authoritative impeachment skeptic, the chief justice has called the constitutional impeachment power "a sort of wild card" that alters the constitutional balance of power by adding another weapon to an arsenal that already includes Congress' right to reject nominees and override vetoes. …