With his speech last night, President Reagan came down from the political stratosphere of last spring's euphoric first round of budget-cutting and into the trench warfare most of his predecessors have faced when they challenged deeply embedded congressional attitudes and interest-group pressures.

The skepticism that greeted Reagan's plan does not necessarily foretell defeat for the economy measure. But it does mean that Reagan and his aides will have to slog it out in a way they avoided in the blitz of Capitol Hill they staged in what now seem the halcyon post-inauguration, post-shooting days.

Both Senate Majority Whip Ted Stevens (R-Alaska) and Finance Committee Chairman Robert J. Dole (R-Kan.) said it would be "very difficult" for Reagan to win congressional acquiescence for his proposals. Compared to last summer, said Appropriations Chairman Mark O. Hatfield (R-Ore.), "there are no rubber-stamps in this Congress."

A lot has happened since those earlier Reagan victories.

Some of the bloom is off the president's personal popularity. There is a wobble in the economy and a greater awareness of the real-world human costs of earlier budget reductions. The Democrats have roused from the almost-hypnotic trance in which Reagan had them slumbering, and some Republicans are nervously aware that a year from now, they will be deep in their reelection campaigns. Interest groups are again more preoccupied with protecting their own turf than helping fight the president's battle. And Congress, that most mercurial of all the governing institutions, is rebelling a bit against the "Simon says" series of rapid-fire commands from Reagan and budget chief David A. Stockman.

In short, Washington is witnessing a return of politics as usual--and it seems unusual now only because it is the first time Reagan has confronted the phenomenon in his brief tenure as president.

The first symptoms of normalcy showed up during the president's August vacation, in the savage and public bureaucratic warfare between Stockman and others on the senior White House staff, on one side, and Secretary of Defense Caspar W. Weinberger over the size of the Pentagon's share of the latest reductions. Stockman and Co. won the first round of the public relations battle, but Weinberger captured the president's support for the minimal figure of $2 billion out of the military budget.

Partly because the Stockman-Weinberger fight delayed the administration's decision making, partly because Reagan was loafing on his ranch while Congress scattered for the August recess, there was a lot of looseness to what had been, earlier this year, a remarkably efficient White House process of consulting key legislators, state and local officials and interest-group leaders.

When Republican lawmakers returned to Capitol Hill after Labor Day, many of them had the shakes about high interest rates choking off hopes for economic recovery. House Republican Leader Bob Michel of Illinois said ominously that "something has to give in 90 days" or the GOP would suffer retribution at the polls.

In this mood, Reagan's abrupt announcement from Camp David late one Saturday that he was holding very close to his original defense figure, followed by an almost offhand presidential statement that scheduled Social Security benefit boosts would be delayed three months, drove many nervous congressional Republicans up the wall.

Rep. Newt Gingrich (R-Ga.), a Reagan loyalist and leader on the first round of budget and tax cuts, complained that his president, like the much-scorned Jimmy Carter, "is making the mistake of thinking that when the White House makes a decision, everyone just falls into line."

When serious consultations belatedly began last week, Republican congressional leaders told the president bluntly that the program he had tentatively suggested "won't fly." Behind the scenes, the pulling-and-hauling got heavy. Rep. Jack Kemp of New News Analysis News Analysis York, chairman of the House GOP Conference and father of the Reagan tax bill, said he would support no further budget cuts until the administration gave serious consideration to a return to the gold standard. A delegation of moderate Republican "gypsy moths," whose votes were crucial to passage of the earlier cuts, told Michel they would not go along unless the Pentagon took more of the new slashes.

Governors and mayors, including Republicans and conservative Democrats who had earlier supported Reagan, reacted with rage at reports of a planned phase-out of general revenue-sharing. Others protested the cuts that would most severely affect their own constituencies.

By coincidence, on the final weekend of administration decision making, the AFL-CIO and a loose coalition of civil rights and feminist groups filled the Mall with 260,000 marchers protesting Reagan's economic policies. Democrats, still smarting from the earlier defeats, rallied to the side of their traditional interest-group allies. Yesterday morning, while Reagan was seeking with only limited success to assuage the feelings of a delegation of governors, Sen. Edward M. Kennedy (D-Mass.) was at the headquarters of the National Education Association, pledging to work with that group to thwart the designs of "the most anti-education administration in our national history."

In this heated atmosphere, Reagan frantically reshaped the final details of his program. The Social Security delay was dropped, the phase-out of revenue-sharing forgotten, but defense stayed where the president had first put it, and the overall size of the package was--at least on paper--big enough to meet Reagan's goal of a $42.5 billion deficit for fiscal 1982.

The governors who had received Reagan's personal apology for the lack of prior consultation still stood up in the White House driveway yesterday and said they would fight the proposed 12 percent across-the-board reduction in most federal grants-in-aid. Members of Congress and interest groups quickly targeted their least favorite cuts.

While most business spokesmen supported the Reagan program, Mark J. Riedy of the Mortgage Bankers Association said the president's effort to reduce federal loan guarantees "adds insult to an already very injured housing industry."

But Reagan showed with his television address, as he had done before, that he was a capable salesman for his own programs. Recognizing the institutional independence of Congress, which just yesterday overrode his wishes on the sensitive issue of U.S. aid to El Salvador, he said, "I recognize that many in Congress may have other alternatives" for cutting the budget, "and I welcome a dialogue with them."

On the politically dangerous issue of Social Security, he abandoned his own earlier proposals, which had got him into hot water, adopted the short-term expedients some Democrats had recommended, and then put the monkey on the Democrats' back by inviting them to join a Republican-dominated bipartisan task force to find a permanent solution to the system's financing problems.

All this showed considerable political deftness, and the early indications were that even previously critical members of the GOP would once again rally to his cause.

From the conservative corner, Kemp said "a lot of the ideas we Republican congressional leaders advanced really were reflected" in the final decisions. "There is a consensus we have to do something further on the spending side, but the mix of defense and domestic is still up for debate."

Gingrich, a fellow-conservative who wants to boost the defense cutbacks, said Reagan "is back to being the leader of the team . . . . He has a good chance of getting it through."

Rep. Lawrence J. DeNardis (R-Conn.), one of the "gypsy moths," said the signals he was receiving from the White House were that "even though they couldn't convince Reagan to recommend more than $2 billion in defense cuts, which is barely scratching the surface, they expect Congress to cut deeper--and it will."

"I think," said DeNardis, "we'll hold together fairly well, but this time it will be the Bob Michel show, not the Ronald Reagan or Dave Stockman show."

In other words, while the president proposes, Congress disposes--which is to say, Reagan is being introduced to politics as usual.