SPECULATION can end on the effect of the first presidential debate: enough post-Oct. 7 polls are in to give a pretty clear answer -- just in time for the final debate. By agreement of just about everyone except Nancy Reagan, Walter Mondale was the winner of the candidates' first encounter; and the polls register some gains for him. In The Washington Post/ABC News poll, for example, Mr. Mondale rose from a pre-debate 37 percent to a post-debate 42 percent -- a statistically significant difference. Also, his post-debate support was notably firmer. The debate enabled Mr. Mondale to increase the size of and firm up his Democratic base. It improved the morale, and hence probably increased the turnout, of core Democratic voters.

But it did not result in massive erosion of the Reagan majority. From the end of August until Oct. 7, Mr. Reagan's percentage -- the single most important result in any poll when you have an incumbent running -- oscillated between 55 and 60 percent in most public polls. In most public polls taken after the debate, the Reagan percentage seems to be oscillating in the 53-to-58 percent range. The sensible conclusion is that Mr. Reagan's support is down, a little. That conclusion is confirmed by results in many statewide polls since the debate.

So Mr. Reagan is not down to the danger zone -- yet. A candidate can win 53 percent of the popular vote and win more than 400 electoral votes, as Franklin Roosevelt did in 1944. Mr. Reagan still has the inherent advantages that accrue to an incumbent president credited with strong leadership running at a time of peace and prosperity.

But a couple of things make the race still tantalizing. One is this Sunday's debate. The other is the fact that, aside from his 60 percent-plus showings in a dozen or so states, Mr. Reagan is running pretty evenly around the country. This is good news for him so long as he maintains his current levels of support: in only a few states have his poll showings dipped below 50 percent. But those include the two biggest states, California and New York, and his showings in several other large- electoral-vote states are only a few points higher. But consider what happens if the president falls from the 53-to-58-percent range to, say, the 49-to-54-percent range, which is where he was in polls against Mr. Mondale from February through June. At that point, as many as 200 hitherto safe electoral votes are at risk (with 270 needed to win).

All this speculation may be academic. But it, or something very much like it, is very much on the minds of both campaigns' strategists as they ponder the debate and the two weeks and two days that follow.