ThE polls this fall are starting to get a little monotonous, but not boring. The latest Post/ABC News poll, based on the enormous total of 12,000 interviews conducted from Sept. 22 to Oct. 2, shows Ronald Reagan with 55 percent of the vote -- essentially the same as the 56 percent he had in the Post/ABC Sept. 6-11 poll. Since late August Mr. Reagan has been getting between 55 and 60 percent against Walter Mondale in just about every published nationwide poll -- a pretty good indication that his support is holding solid in the high 50s, well above the magic 50 percent level.

This steadiness, in these days of volatile electorates and allegedly unreliable polls, is notable itself. Mr. Reagan's current 55 to 60 percent support represents a ratchet upward from the levels he enjoyed before Feb. 1 (49-54 percent) and from February through June (50-55 percent), before the results got buffeted around by the conventions. It doesn't necessarily foreshadow a landslide. It's lower than the 60-plus levels that landslide winners Richard Nixon and Lyndon Johnson were getting in polls through most of the 1972 and 1964 campaigns.

At the same time, Mr. Reagan is notably stronger than the last two incumbent presidents who ran. By this point in the 1976 campaign, two debates had taken place, and President Ford, once far behind, had gotten to within a few points of challenger Jimmy Carter. But he didn't exceed the 48 percent of the vote he ultimately won.

As for 1980, who can forget the widespread surprise when Ronald Reagan did so much better on Election Day than he had been doing in the polls. But the polls had shown, consistently from early 1980 to October, Mr. Carter running at or below 42 percent -- a devastatingly low level for an incumbent president. Hs standing rose higher in some polls in October, as the support for independent John Anderson evaporated. True, Mr. Reagan's lead in most polls was unimpressive, and in some he trailed Mr. Carter. But the Carter numbers were the key. Voters were prepared to reject him if the alternative proved acceptable, as Mr. Reagan did after the Oct. 27 debate.

So for the next five weeks, keep your eye not on Mr. Reagan's margin but on his percentage. And watch for the two separate things that have to happen in voters' minds for that percentage to drop. First, voters' reservations about Mr. Reagan -- and they have reservations, as they do about every politician -- have to be strengthened until they overshadow the current levels of satisfaction with his foreign and domestic policies and leadership style. His kitchen repairs and intelligence comments may have done something to undermine the view that he is a strong leader, but obviously they don't weigh heavily enugh to change the balance. Second, Walter Mondale must overcome the reservations voters currently have about him if he wants to hold Mr. Reagan under 50 percent. This may be easier, since impressions of the incumbent are much more firmly founded than those of his challenger.

What are the odds of both these things happening? You could get numbers at a London bookmakers. Much more interesting is to watch what these two talented, strong partisans do over the next five weeks.