Washington — Ronald Reagan is entering the most critical part of his presidency, White House insiders and outside experts say.
The character of his presidency will be sharply etched, aides say, by budget decisions made before the Christmas holiday. From all reports, a tight-fisted budget is likely to emerge, squeezing hard on programs from health care to national parks.
''This is the ball game for him,'' says White House scholar Thomas Cronin of Colorado College of the budget-decision period President Reagan has entered. Reagan's buoyant confidence helped elect him and kept him afloat, Mr. Cronin says. ''Confidence in a leader is contagious,'' he says. ''But public patience is running out.''
Reagan's immediate decisions could help tip the political scale in Washington one way or the other and largely shape the 1984 election.
Insiders are not sure whether Reagan's basic budget strategy with Congress will prove confrontational or accommodating. Some in the White House think he may send to the Hill early next year a budget that leaders of both parties will reject out of hand. If Reagan does not soften his position on defense spending and tax policy while pushing for greater spending cuts, they say, he could lose the initiative in the capital on which his image as a national leader largely depends.
''Since 1968 it's been a Republican White House era,'' Cronin says, ''with the accident f Carter's election due to Watergate. That's a 16-year period. Maybe it's time for a Democratic era''
On Capitol Hill, Democrats were pleased that they met their major goal for Reagan's first two years - preventing a political realignment which would have resulted in a Republican majority. No such realignment is on the horizon, White House aides concede.
Reagan himself seems in better shape to run in 1984 than does his party, White House insiders say. He might be able to finesse an impasse with Congress over the budget and other issues. But his party - most notably GOP Senate candidates - seems poised for a 1984 rout if the general political and economic climate doesn't improve by next summer.
Reagan is expected to finish shaping the new budget by the Christmas recess. One White House camp sees Reagan in danger of hemming himself in with the budget. They keep citing, as a positive precedent, his pattern as California governor. In that office, he reversed his field on practical matters as long as he could still affirm his ideological goals. They fear Reagan could align himself too far right of center, leaving the center and center-left to Democrats in the 1984 race.
This group boosted its case last weekend with release of a trial projection by White House Office planners of how the nation's 538 electoral votes might split among Reagan and two likely Democratic challengers, Sen. John Glenn of Ohio and former Vice-President Walter Mondale.
Senator Glenn, regarded as a Democratic centrist candidate, led Reagan 243 electoral votes to 212, with 83 electoral votes in doubt. Mr. Mondale, a center-left candidate, trailed Reagan 186 to 231, with 121 electoral votes in doubt. A total of 270 votes are needed to win election.
GOP officials reacted with surprise and dismay at circulation of the White House '84 assessment. ''It's too early to begin trial heats,'' says one GOP official. ''After six more months of governing, then you can begin trial heats on a national basis. By saying now that Reagan trails Glenn and narrowly leads Mondale, you're telling Congress . . . we're already looking at a lame-duck presidency. I don't think that's true.''
In previous budget strategies, Reagan pushed hard for more than he thought he could get, anticipating Democrats and moderates would come around later. ''I would hope the (White House) decision on negotiations with Congress will be made in terms of what is realistic rather than on what would be desirable,'' says one GOP professional.
''I don't see any good coming from taking a tough line and then compromising back,'' he adds. ''The argument you can adjust a little here and a little there without destroying the nature of a program has run its course legislatively.''
What would be gained from making trade-offs up front, White House aides say, is that Reagan would be spared publicity for seeking cost-cutting measures that could be blocked by Congress later anyway.
The first round of White House budget decisions concluded Dec. 12, when the President decided to seek $25 billion to $30 billion in nondefense cuts. Following that, budget director David A. Stockman sent discretionary spending outlines back to the departments and agencies, which responded last week with their preferred budget versions. Many of the differences between Mr. Stockman and the agencies were settled directly. Others went to the White House Budget Review Board, headed by Reagan aides Edwin Meese III and James Baker III, and including Stockman, for review. The final budget phase is headed by President Reagan himself. In it, appeals that could not be settled earlier are resolved and big issues, like any defense and tax priorities, are taken up.