President Reagan's reluctance to adapt fully and quickly enough to changing economic circumstances could soon lead to a challenge for the Republican nomination, GOP professionals say.
Democratic hopefuls are already on the campaign trail. Next Wednesday, Sen. Alan Cranston of California is scheduled to be the first to announce his candidacy. Mr. Reagan has ''two or three months'' to avoid an open split in the ranks of his own party, Republican strategists say.
The Democrats have sketched in their own 1983 and '84 timetable. During the first six months, they'll set up a legislative platform that portrays them as a party of alternative ideas. Key Democrats on important congressional committees will initiate or shepherd through bills to fill out a Democratic agenda through next fall. In early '84 the presidential campaign will be in full swing. The Democrats ambitiously hope to have a comprehensive program - featuring tax reform, research and development, education, energy, defense, and arms control - backing up their contenders.
Members of both parties have concluded that Reagan may rhetorically be signaling change, but is actually hewing tightly to the policies and principles of his first two years. Bipartisanship is apt to prevail, but it will prevail more among lawmakers on Capitol Hill than between Capitol Hill and the White House, legislators say. Both parties want some record of progress to run on in 1984, and they fear a stalemate with the White House could impede that progress.
The Democrats remain handicapped by the lack of a single spokesman, their GOP counterparts say. Democrats counter that their congressional effort, featuring people who aren't active candidates, will give their presidential contenders more time to get better known before coming under attack for specific proposals.
But it's the warning of a challenge developing within the President's own party that political professionals here, who asked to remain anonymous, find most arresting.
''Reagan waits too long, responds too little,'' says one GOP official, a veteran of Reagan campaigns. ''He is creating a situation where the patience of those he must lead is diminishing.''
Another Republican official - a Reagan loyalist - puts it more bluntly: ''I have felt for a long time that Ronald Reagan was going to be a one-term president. What Ronald Reagan is trying to establish is his role in the party as the person best able to articulate the philosophies and ultimate goals of Republicans.
''He will compromise, as he did in California when dealing with the Democratic Legislature. But his compromises will come very late and they will be the absolute minimumsm - just enough to squeak through.
''He is going to hold the course, with the same supply-side emphasis - keeping taxes in line and cutting them this next fiscal year, increasing taxes only if Congress comes along with some other cuts in spending.
''He doesn't care whether it plays politically or not. The concern is he's going to face a tremendous backlash internally. In the Senate alone you have 18 or 19 guys who went with him his first two years. But if they don't see significant changes of direction the next two or three months, they will be openly advocating some splits and going their own direction.
''Without significant change there will be some open wounds in the party. There could very well be a challenge.''
Sen. Bob Packwood (R) of Oregon is seen as one Republican prospect who takes issue philosophically with Reagan.
Whether Reagan will run is not known. Republicans do not expect him to announce until fall.
''My own inclination is to say he will not run,'' says a Republican official. ''We may be looking at somebody off the bench - Howard Baker, Bob Dole, Jack Kemp.''
Democrats observe that any GOP challenge to Reagan for the party's nomination would likely be squelched if Reagan wanted to run again. But it would hurt him.
''The nomination process for the Republicans is much more in the hands of Republican regular types,'' says Rep. Don J. Pease (D) of Ohio. Democratic challenges to a standing president have a better chance of succeeding.
''In terms of the success of a challenge, the two parties differ,'' Mr. Pease says. ''If you get a Gene McCarthy opposing a President Johnson, a McCarthy appeals to a significant percentage of people who are active in Democratic primaries and conventions - liberals, activists, labor, and so on. They can cause significant damage in the nomination process.''
A challenge by a GOP moderate like Mr. Packwood, who would have a smaller base among Republican Party activists, ''would play on the theme of dissatisfaction with Reagan,'' Pease says. ''It would further identify the Republican party with reactionism, nonprogress on women's rights.''
Reagan's standing on Capitol Hill has altered in recent months. ''I detected, especially in the welcoming applause, a sense of false enthusiasm [at his State of the Union address in the House chamber], among the Republicans as well,'' Pease says. ''If the polls continue to show the public is reaching the end of its string with Mr. Reagan, he will be perceived by the end of this year as a major liability by 1984.''
Says Paul Maslin, a pollster for President Carter and active with Patrick Caddell's firm, Cambridge Survey Research: ''There's a feeling on the Hill - among Democrats and Republicans - that Congress is ready to go one step further than Reagan. That's where the bipartisanship is - not for Reagan's specific proposals.''
The need to react responsibly as a party of ideas shaped the Democrats' rebuttal to Reagan's State of the Union message.
''It was the best thing to have happened to the Democratic Party in years, if not the decade,'' says Stuart Eizenstat, Carter's chief domestic adviser and now a Washington lawyer active in Democratic affairs. ''It represents a real beginning of a consensus for an alternative program which is based on dealing with the new structural problems of the economy - and yet in a responsible way, without interjecting government into everything.''
''For 1984, this may be the first time presidential candidates pick up a program from the congressional wing, rather than the other way around,'' Mr. Eizenstat says.