Much at stake in candidate debates.

Will the debates do it? Democratic strategists are relying heavily on the October presidential debates to bolster the image of Walter Mondale and invigorate his still-struggling campaign. Far behind President Reagan in the polls, the Democratic presidential challenger is now trying to define the election issues more sharply. He is also trying to attack the Reagan administration more forcefully in an effort to put the President on the defensive.

''The debates are critical to the success of the Mondale campaign,'' says Michael Steed, executive director of the Democratic National Committee. ''Especially as long as the President remains as isolated as he is and refuses to address himself to the future of the country.''

But some political experts caution that Mr. Mondale will need something more than the debates if he is to pull ahead of Reagan.

They argue that an incumbent has an advantage in a debate, in the sense that he ''wins'' if he does not lose or even if there is a perceived tie. So unless Reagan does poorly - and that is unlikely, judging from past debates in which he has participated - Mondale cannot count on these two single events as a make-or-break proposition.

''Mondale needs to get to the stage of the debates having framed the issues is such a way as makes people unsure about Reagan,'' says Norman Ornstein, a political analyst at the American Enterprise Institute. ''If, for instance, he can make it stick that Reagan is dangerous - because of his arms policies and the deficit - the debates provide the possibility of reinforcing that (charge). But he has to make the issues work first.''

The big face-offs take place in October under the auspices of the League of Women Voters. The first, to be held Oct. 7 in Louisville, Ky., will deal with economic and domestic issues and the second, on Oct. 21 in Kansas City, Mo., with foreign policy questions. In between, on Oct. 11, Vice-President George Bush and Democratic challenger Geraldine Ferraro will square off in Philadelphia - an event expected to generate as much public interest as the presidential debates, if not more. To show how conscious the candidates are of good television exposure, the debates were scheduled so as not to conflict with the World Series baseball games.

Mondale would have liked as many as six debates. President Reagan, who is running ahead in every region of the country and therefore has no real need to debate, would have preferred none. But nowadays everyone, including the news media, has a vested interest in debates, and it is difficult for a president to avoid them without appearing to be afraid or isolated from the political fray.

Political analysts suggest that the President has a competitive spirit and does not want it perceived that perhaps he is not smart enough to debate.

Reagan did decline to have a direct exchange with Mondale, however. The Democratic candidate and many political experts believe a one-on-one confrontation offers more possibility for insight into each person's thinking and for gauging leadership traits. But the President's political advisers do not like to put Reagan in such spontaneous situations, where there is more leeway for embarrassing mistakes. With a panel of journalists questioning the two men, Reagan can be well briefed on likely questions and better prepared for the debates, each of which will last 90 minutes.

Reagan campaign officials voice confidence that the President will do well in the debates but stress that the debates are not critical. ''We expect people to make up their minds based on the experience of one or the other in their respective positions,'' says John Buckley, deputy spokesman of the Reagan-Bush ' 84 committee. ''They are as defined as any two men, and there is no new definition that will arrive out of those debates.''

While it is not easy for a presidential challenger to take on an incumbent, especially a popular one, the potential for gaining some advantage is always present. In the 1980 campaign, Reagan debated President Carter and emerged with public perceptions that helped him in the election: One, he held his own with the President, and, two, although Carter tried to portray Reagan as a warmonger, he appeared as a reasonable individual.

Political experts say that the debates could help Mondale politically just as they did Reagan in 1980. ''The debates could be the key because they can show leadership, decisiveness, mastery of the issues - the things which are concerning people now,'' says Frank Mankiewicz, a veteran Democratic operative. ''So it's not just a question of winning the debate and catching Reagan in some blunder, which would be helpful. Kennedy pulled himself up in 1960 by showing self-possession, that he was not just a young guy that knew nothing.''

As for the Bush-Ferraro confrontation, a widely held view in this town is that the vice-president will not have an easy time of it. He may know a great deal more than Ms. Ferraro in such crucial areas as foreign policy, and he has substantial experience in high office. But, say observers, his ''preppy'' image and style may suffer when contrasted with the down-to-earth, no-nonsense demeanor of his opponent.