Two almost unprecedented actions on Capitol Hill last week signal a power shift that very likely will prove to be of historic dimensions.

In one, the Senate Republican leadership, under the new majority leader, Bob Dole of Kansas, announced that it would prepare its own budget proposal and have it ready for consideration on Feb. 1, three days before President Reagan is scheduled to present his budget.

So far as I could check, it is unprecedented for the leaders of either house of Congress, linked by bonds of party loyalty to any president -- let alone one who has just won an election landslide -- to draft their own budget for the executive agencies, without even giving the president the courtesy of waiting for the submission of his spending blueprint for the year.

The second event, which occurred the same day, was the decision of the House Democratic caucus to unseat 80-year-old Rep. Melvin Price of Illinois as chairman of the House Armed Services Committee and replace him with the seventh-ranking member of that committee, 46-year-old Rep. Les Aspin of Wisconsin.

Only four times before had the House breached the seniority system to remove a committee chairman, the last time a decade ago. Never before had it dipped so deeply into the ranks to find a successor.

At the most obvious level, the two actions send a searingly clear message to the lame- duck leaders of the two parties and the two branches of government, 73-year-old President Reagan and 72-year-old Speaker of the House Thomas P. (Tip) O'Neill Jr., that their days and their influence are both waning.

Reagan had been warned repeatedly by leading Republican senators and representatives, including some of the most conervative men in both chambers, that his budget would be DOA -- dead on arrival -- if he did not impose significant spending discipline on the Pentagon as well as the domestic agencies. Reagan ignored the advice and went along once again with Secretary of Defense Caspar Weinberger.

While White House aides say they welcome the Senate Republicans' going off on their own, the spectacle of the president being bypassed on the budget sends a clear message of executive failure on one vital area of policy.

O'Neill was also warned that Price's physical infirmities had reached the point that they impaired his leadership of the important Armed Services Committee. He was urged to arrange a graceful exit for Price, perhaps as chairman emeritus. But O'Neill, who is retiring himself in two years, tried to procure another term for Price -- and was repudiated by his membership on the first test vote of the year.

At a deeper level, both actions indicate where the power is moving in Washington: from the White House to Capitol Hill and, within Congress, to a new set of leaders, relatively unencumbered by the thinking and the loyalties of the past.

The loss of energy and leadership that is a chronic threat to any second-term president has struck Reagan forcibly even before his second inaugural. The exodus of key aides, which was predicted in this space a week ago, is coming even faster than anticipated, with Michael K. Deaver and William P. Clark, two longtime associates of the president, leaving the administration, Donald T. Regan and James A. Baker III, two of the top recruits for the first term, shuffling jobs, and a general sense of scrambling to fill the open slots.

Reagan still has important cards to play. With major speeches coming in the next month and veto threats available later in the year, he can influence Congress as it digs into its vital decisions. But his willingness to go before the cameras Tuesday and Wednesday for an announcement of the personnel changes and a press conference demonstrated his own awareness that he becomes almost a cipher in his own government when he is not seen on the little screen, playing president.

The people who are filling the power vacuum are people who expect to live the most important part of their political lives in the post-Reagan, post-O'Neill era of the late 1980s and 1990s. Dole has just taken over as Senate leader and already has established a greater degree of independence from the White House than his precedessor, Howard H. Baker Jr., ever demonstrated. He can do so in part because so many of the key committee chairmen -- like Pete Domenici on Budget and Bob Packwood on Finance and Mark Hatfield on Appropriations -- are also men of great independence and political freedom.

In the House, the shift is even more dramatic, because it is clearly a generational change. Aspin came to Congress in 1970 after Pentagon service during the Vietnam War. The new chairman of the House Budget Committee, 42-year-old Rep. William H. Gray III of Pennsylvania, was first elected in 1978. The new chairman of the Democratic caucus, which put Aspin and Gray into their positions, is 43-year-old Rep. Richard A. Gephardt of Missouri, who has been in Congress only since 1976. The only member who spoke in the caucus for removing Price, in defiance of Tip O'Neill, was 34- year-old, third-term Rep. Dave McCurdy of Oklahoma.

Power is shifting. In your lifetime and mine, it will not likely shift back.