IT WAS A fine and spirited exchange that President Reagan and Walter Mondale had in their first campaign debate last night. The differences in their personal styles and their approaches to the nation's economic and social problems came out sharply. Certainly prospective voters who had not made up their minds, or whose minds are still open to change, could have found plenty of material to bring them down on one side or the other. We say this despite our conviction that still more could have been learned if the journalists, skilled as they were, had been no part of the picture. Neither Mr. Reagan nor Mr. Mondale needs an intermediary.

Mr. Mondale approached the evening feeling it was his last chance to start narrowing Mr. Reagan's seemingly insurmountable lead in the polls and to convince voters that the choice still open. Overall, he was, we thought, impressive, a man who had finally found both his authentic vigorous personality and his appropriate theme: the quality of the future that Americans should want to build. This time around, he pleased his supporters. He was clear and logical, gracious and forceful in turn, and he succeeded in bringing the debate regularly back to what he defined as the central issue of the campaign: the budget deficit, its economic and social implications, and what it shows about President Reagan's command of the reality around him.

Plainly, President Reagan was uncomfortable in addressing the less defensible human and economic consequences of his budget cuts and his tax cuts, and he shunted the discussion of these matters to a side track a number of times. Still, Mr. Reagan showed again that it is very hard to lay a glove on him. His qualities of good humor and sincerity and his talent for quick and sometimes misleading statistics (on poverty, for instance) and the smooth glide to a familiar, invariably optimistic theme were there in abundance.

Mr. Reagan, in reviewing his own record in favorable terms, was going over terrain familiar to the voters and pleasing to his supporters. Mr. Mondale, trying his best to distinguish himself in as many ways as possible from the president, was able to give the clearest statement he has made so far in this campaign of what he thinks is demanded of the presidency at this time. The campaign is a lot more interesting as a result of this debate.