For Ronald Reagan, the twilight days of an extraordinary presidency approach. He retains, of course, the immense war-and-peace powers granted to any American president. Daily events in the Persian Gulf demonstrate how great those powers remain. He continues to be influential in world affairs. A chance to achieve a significant arms control agreement with the Soviet Union still exists. But in other areas his political powers clearly are waning.

Nothing more dramatically underscores the diminution of Reagan's influence than his role in the increasingly embittered, and in the end destructive and hopeless, effort to win Senate confirmation for his Supreme Court nominee, Robert H. Bork.

Future scholars of Reagan's presidency looking for a symbol of how the tide ran out on the Reagan era may point to this week, when the three major television networks, ABC, CBS and NBC, refused to broadcast Reagan's speech in which he attempted to rally public support for his nominee by strongly attacking critics of Bork.

Here was the Great Communicator, the quintessential television president whose performances over the years enabled him to win battle after battle by carrying his case directly to the public, figuratively struck mute in one of his most important political struggles. The networks had deemed his message unworthy of live news coverage.

Even had his speech been broadcast, there is no reason to believe it would have made the slightest difference. The public seems to have tuned out Reagan, just as the Congress increasingly treats him as irrelevant. The right-wingers, too, who hailed him as their messiah, have distanced themselves from him of late. Predictably, they have fallen to fighting among themselves -- and to attacking Reagan as well for what many of them view as his too-little, too-late, too-tepid handling of the Bork defense.

So, speaking almost as if offstage, Reagan sputters defiance, pops off, ad-libs, engages in excessive political rhetoric ("if the campaign of distortion and disinformation used by opponents of this nominee is allowed to succeed . . . it will permanently diminish the sum total of American democracy") and plain nonsense (that the Bork battle really is about "the independence of our judiciary") -- and all to no apparent effect.

Another incident this week contributed to the sense of Reagan's growing isolation. It was Labor Secretary William E. Brock's quiet departure from the Cabinet. Brock, who initially served as the president's first trade representative, is one of the 17 original Reagan Cabinet members.

Now only two of those Cabinet officers are left: the still highly visible and forceful defense secretary, Caspar W. Weinberger, and the still nearly invisible and almost publicly unknown housing secretary, Samuel R. Pierce. The roll of the others now gone tolls the passage of the Reagan years: CIA Director William J. Casey and Commerce Secretary Malcolm Baldrige, both of whom died this year; Secretary of State Alexander M. Haig Jr.; Attorney General William French Smith; Treasury Secretary Donald T. Regan; Interior Secretary James G. Watt; Budget Director David A. Stockman; United Nations Ambassador Jeane J. Kirkpatrick; Transportation Secretary Drew Lewis; Labor Secretary Raymond J. Donovan; Health and Human Services Secretary Richard S. Schweiker; Agriculture Secretary John R. Block; Education Secretary Terrel H. Bell; Energy Secretary James B. Edwards.

The same dramatic turnover has occurred among the president's senior White House staff. His original team included the Big Three, or Troika: presidential counselor Edwin Meese III, chief of staff James A. Baker III, deputy chief of staff Michael K. Deaver, all of them gone from the White House. Gone, too, are the rest of that first White House crew: press secretary James Brady, domestic affairs adviser Martin Anderson, national security adviser Richard V. Allen, economics adviser Murray L. Weidenbaum, political adviser Lyn Nofziger, congressional liaison Max Friedersdorf, public liaison Elizabeth Hanford Dole. The problems of Deaver and Nofziger, whose criminal trials approach, convey another symbol for the Reagan years -- the ethical impropriety affecting many who held high positions during his two terms.

Reagan stands virtually alone today, and for him, the inexorable political countdown begins. Nine months from now, the political parties begin nominating their next presidential candidates. Twelve months from now, the next president will be elected. Then Reagan, too, will be gone. All that will be left is the reckoning on his presidency, and a complicated one it will be.