IT'S NOT the same set of voters who turn out and vote every four years. Americans go to the polls when they're enthusiastic about their choices and stay home when they're glum about them, and the mix changes. Walter Mondale's showing in the first, Louisville debate, if it did nothing else, toned up Democrats' morale and hence probably increased Democratic turnout. The pollsters can't measure that effect, but in marginal Senate or House races it could be worth 1 or 2 percent. Neither the Philadelphia debate nor the one Sunday in Kansas City may have generated the voter enthusiasm to make a difference in the turnout on Nov. 6.

In House races, dozens of Republicans will be hoping to overtake Democratic incumbents in the next few weeks. Mr. Reagan's performance in Louisville had foreclosed any chance of a mid-October offensive by the president in behalf of such candidates. His performance in Kansas City restored his luster to the majority of voters currently favoring him.

But did he give voters reasons why he needs more Republicans on Capitol Hill in the next four years? His message on both domestic and foreign affairs is that he wants to continue on the course of the past four years. But that course has been set, in part, by the Democratic majority in the House. We saw little in what the president has called for that could not be achieved with a Congress like the pres

Mr. Mondale is the standard-bearer of a party whose candidates have consistently run on their own issues and personalities. Few voters seem to impute to them the positions, good or bad, they see Mr. Mondale taking. For the next two weeks Republican ads will attempt to tar Democratic congressional candidates with Mr. Mondale's tax-increase proposals. But the effort may be undercut by the jibes from the Reagan campaign about how virtually no Democrat has endorsed them.

While the debates focus voters' choices in the presidential contest, they seem likely to have a limited impact on the 33 Senate and 435 House contests going on simultaneously -- but more or less autonomously. The debates have left both parties' backers with reasonably good morale, but they have left the burden of most local campaigns on the candidates themselves.