The growing perception in Washington is that President Reagan is going to win his battle over the budget -- even though it is clear that he won't get all he wants from Congress.
The political observers making this point assert that Mr. Reagan's advantage stems from his ability, as veteran congressional observer Bryce Harlow notes, ''to keep Congress preoccupied with his budget.
''They're fighting on his turf,'' says Mr. Harlow, one-time counselor to President Nixon. ''Basically, Congress is adjusting to the President's program -- to his initiative. It's the President who is controlling the agenda of the country.''
When asked about this argument, White House advisers agree Mr. Reagan has momentum. But at this point the White House is still concerned that the diluting of the President's budget going on in the House may be too great for him to accept.
The White House seems likely to be taking this political position to ensure that the President gets all the concessions he possibly can from the House before the start of the Senate-House conference hearings that will hammer out the final budget.
The White House still warns that a presidential veto is a possibility -- and that Congress should not forget that Reagan is ready to use it.
Already, the President is saying that he has had to give up too much - fewer spending cuts and a smaller defense buildup.
''Sure Reagan must give up a lot,'' argues Mr. Harlow. ''But if as a result of the budget being put in place, the economy improves and people begin to feel better, the President can say: 'Look what I did.' And it will play.''
That Reagan is still in the driver's seat, although far less comfortably than last year, is evident in these developments:
* There has been a coming together in the last few days -- at least much closer than it has been for months -- of the coalition of Republicans and conservative Democrats in the House which shaped Reagan victories in Congress last year.
Reagan's slip in public opinion polls had appeared to make that coalition an impossibility for this year's budget battle. But the President's call to Americans to prod their congressmen to vote for the budget he favors has brought results.
Thus, the prospect that Reagan will get his bottom-line budget, containing concessions that he reluctantly has agreed to, is much brighter as the showdown votes draw nearer.
* Even some liberal Democrats are growing uneasy over waging a fight to the bitter end to keep the President from getting a budget near what he's asking for.
They, too, have been back home of late and have found out that, despite the flagging economy, Reagan still remains well liked -- and that a lot of people think he should have still more time and congressional support in his effort to right the economy.
Thus, some House liberals are talking as though they might vote for the Republican budget supported by the President -- if a few more concessions can be made to them. These would include more money for medicare and some other social programs.
Further, much as happened last year, there is growing talk among liberal Democrats that they might go along with Reagan so as to saddle him with his own program -- a budget that, they say, will turn out to be a distinct failure and a political liability to him.
Meanwhile, a Republican opponent of Reagan during the 1980 presidential primaries -- John B. Connally -- told reporters over breakfast on May 26 that he thought the President was ''basically on the right course.'' He added that ''Reagan has all the problems any President would have when trying to change a country's track.''
Former Texas Governor Connally says the President is having real difficulty in giving the country's economy a lift as long as Congress ''refuses to tackle entitlements, particularly social security.''
But he says he feels Reagan has some momentum, that the President is ''tough-minded in his support of principles,'' and that he is moving ahead, though more slowly than last year. Reagan's problem, says Connally, is that ''Americans are not a patient people.''