I paid a visit to my new sports coat this morning. It's woolen and heavy, a tweed befitting a thinker such as myself. I bought it almost a month ago thinking that the weather would soon turn cool as it always does this time of the year. I wore it once and almost died of heat prostration.

I know that I am not the only one looking longingly at clothes newly bought for the winter. A friend of mine bought a suit, winter variety, and just couldn't stand looking at it in the closet. He wore it last week, one of those 80-degree days, and still talks of the experience.

We are all suffering. The talk in Washington is not of the presidential campaign, not of Walter Mondale and Ronald Reagan, but of the weather. For a couple of days a while back we had a touch of fall, and then summer returned in all its Washington awfulness. It's incredibly muggy, the grass behind my house is still growing and the mosquitos are back.

I have a theory about all this. I know that if I bothered to watch one of the television weathermen, one of those cap-toothed guys who's asked to pretend that the weather is somehow his responsibility, I would be told that the jet stream has shifted. This is their way of saying something has happened which is what we already know. For a while, you may remember, everything was blamed on El Nino and before that it was atomic-bomb testing in the Nevada desert. Now it is the jet stream. Believe none of this. It is Michael Deaver.

Deaver, as you know, is the guy in the White House who plays Ronald Reagan like a pipe organ. He pulls the stops and presses down on the pedals and, lo and behold, sounds come out of the president. Like Cecil B. DeMille and Busby Berkeley before him, Deaver is a master of the extravaganza. He is the executive producer at the White House, the one who broght us the president at Normandy ("The Shortest Day") and arranges the president's schedule so that the business of governing does not smother the business of leadership.

Deaver must have noticed that Mondale started to edge upward in the polls when the weather turned cool. In fact, during the recent cool spell, Mondale went from about 20 points back to around 10. Had the weather held, Mondale might today be neck and neck with the president. It was then that Deaver acted.

I would like to be able to tell you that I know what Deaver did. I do not. All I know is that summer returned and Reagan shot up in the polls again. Bear in mind that this happened right after the latest Beirut bombing. Reagan first blamed that on no one in particular, later settled on the Carter administration, and then all but conceded that after almost four years of yelling about terrorism the administration had no policy on it. You would have to concede that Reagan's risen the polls is downright inexplicable. The only explanation can be the weather. Reagan is a hot-weather politician.

Mondale, as a Minnesotan of Norwegian extraction, is not. The man seems to perform best when the temperature is either cool or cold, which is what many Americans think he himself is. Given a good string of days suitable for ice fishing, Mondale would be writing his inaugural address.

But even aside from that, it's clear that hot or warm weather puts voters in a summer frame of mind. They slow down. They tend not to pay attention to things. How else can you explain that the president promises to give the Soviets America's Star Wars technology -- and he shoots up in the polls. Some people are probably still waiting for the World Series.

So, however he has done it, my hat is off to Deaver. Summer is back, and with it the message that America is also. The jet stream has moved or El Nino is still causing trouble. Believe that if you will, but in Washington we know who's responsible.

Soon, though, the weather will turn cold. I will be able to wear my sports jacket, the one the moths are now feasting on, and people will start paying attention to politics. By then it will too late, and Ronald Reagan will again be president. It will be the winter of our discontent.