Louisville, Ky. — Ronald Reagan may have to prepare harder for the second presidential debate than his political handlers had counted on. When the President walked off the stage following the first debate with Walter Mondale, the audience gave him a respectful applause. When, moments later , the Democratic challenger strode off into the wings, enthusiastic cheers broke out in the hall.
Most of the cheering was partisan. Democrats - party leaders, senators, congressmen - were wearing smiles as they emerged from the Kentucky Center for the Arts where the debate took place. Clearly they are heartened.
''Mondale came across as stronger, more in control, and Reagan seemed to struggle,'' commented Rep. Tony Coelho (D) of California. ''That means Reagan can't focus on the Senate and House races. He has to prepare for the next debate.''
But professional media experts likewise say that Mr. Mondale did extemely well, showing himself fully able to hold his own with the President.
The net effect of the first debate, experts say, is to solidify Mondale's own supporters, who had become almost embarrassed by his poor image. But it is thought unlikely that Mondale has cost Reagan his huge lead in the polls - or the election in November.
To the surprise of many, however, Mondale was as strong on style as he was on substance. They cite these pluses:
* He seemed more at ease than Reagan.
* He was gracious to the President even while forcefully making his points. (''I like President Reagan,'' he said at one point).
* He showed he had a sense of humor, pulling off a remark that drew the biggest laughter of the evening.
* Mondale's voice, often described as ''whiny,'' was less so on this occasion.
* Instead of conveying gloom, as some perceptions have it, he was more upbeat.
No new ground was plowed as the two men fielded questions from a panel of three journalists on such issues as budget deficits and tax increases. But because expectations were higher for the ''Great Communicator,'' Mondale seems to have gained at least a tie if not more than that.
''Given that the two have different capacities, Mondale performed more up to his capacity than did Reagan,'' says Austin Ranney of the American Enterprise Institute. ''Mondale came across as knowledgeable, pleasant, firm. Reagan was hesitant, a little defensive, but still came on as an attractive guy who did not do himself great damage.''
''Reagan was on the mark,'' says Michael Robinson of George Washington University. ''He was reasonably informed, reasonably inarticulate without a prepared text, warm, and with a conservative message that he reiterates effectively despite his syntax. So it is an irony that Walter Mondale should have bested him on style.''
Comments communications specialist Kathleen Jamieson of the University of Maryland: ''Mondale came in with low expectations, and that was an advantage. We are accustomed to seeing Reagan in scripted performances, and people have not seen that side of Reagan.''
Political experts suggest that an incumbent inevitably ''loses'' a debate because he allows his opponent to cross the threshold of respectability, which Mondale did.
Reagan did not make any major blunders. But it is thought that his strategists will try to tighten up the President's approach in the second debate , given its importance to the Democratic challenger.
''Mondale must capitalize on this and hope than he can use the second debate to reinforce the concerns of people about Reagan and his foreign policy,'' says political scientist Norman Ornstein.
The next presidential debate sponsored by the League of Women Voters takes place in Kansas City on Oct. 21. It will deal exclusively with foreign policy, the area where Reagan is deemed weakest.
The political professionals now see Mondale starting to play the theme that the President is ''out of touch.'' He did this with finesse Sunday night, says Professor Jamieson, by suggesting that the President genuinely believes that the poor are better off and that the test of leadership is whether one sees the problems clearly.
''Mondale's learned a lot about past Reagan debates, and he's skilled at interpersonal diplomacy,'' says Professor Jamieson. ''He framed his points against Reagan very diplomatically.''
If Mondale is perceived by the American public as having done well - and that remains to be seen - he did so on the toughest issues.
''Mondale was more at ease and had a more immediate grasp of the facts over Reagan,'' says Martha Kumar, a media expert at Towson State University. ''Reagan seemed to be lost at times and looking for words.''
Substantively, the contenders reiterated well-known positions on deficits, social security, abortion, religion, and social services. Prodded repeatedly by Mondale to say what he would do in a second term, the President fell back on running on his ''record.'' Analysts think this may have been his biggest ''error ,'' together with unsuccessfully harking back to the Carter-Mondale administration whenever Mondale addressed the future.
Experts give Mondale the edge on his closing statement. The general perception is that it was coherent and all of a piece, whereas the concluding remarks by Reagan, who ordinarily does well in a stem winder, seemed unfocused.