THERE are several big mysteries in Washington these days.House Speaker Tom Foley pointed one out the other morning when he told reporters over breakfast that the chief enigma of the day, to him, was the lack of public excitement over the agreement on the first treaty of the atomic age to actually reduce the Soviet and US arsenals of long-range nuclear weapons. It's true. The sudden warming after decades of cold war and moves by Gorbachev toward accepting a market economy somehow haven't evoked the kind of public stir one might have expected. My recent trip to the Midwest found the conversation focused on crops and the economy and, still, on the glories of the Gulf war - but not on the almost-overnight melting of the global ice. It is a mystery. A related puzzle is the failure of anyone, even savvy politicians, to take note of what might well be entitled, "The Disappearing Issue." For years anticommunism was the meat and drink of the hard-core element within the Republican Party. Indeed, the issue attracted many conservative Democrats, particularly in the South. But that issue, based on the fear that Soviet communists were intent on taking over the world - and, particularly, us - is gone. The breeze out of Russia may not yet tell us of Western democracy taking hold in that country. But everyone knows that great change is under way and that leaders there are preoccupied with their own domestic problems, not with plans for aggression abroad. Yes, there are differences of opinion in the US on how to deal with the changed Russia - how much help to give Gorbachev and how to read what actually is taking place. But these differences do not neatly constitute an issue backed by conservatives and opposed by liberals. Another, related mystery is why so few have noticed that without a communist issue to separate them, America's two parties have grown closer together. With so many Democrats pulling away from the old liberal approach to solving problems (big spending for big government programs), the parties are sounding more and more alike. The Democrats want to get away from a "me too" image. But news out of Democratic camps - that Democrats are coalescing on a '92 strategy that will focus on middle-class anxiety - doe sn't give them much of a distinctive issue. The Republicans have been making their appeal to that same middle-income group for years. Finally, there are the mysteries about who is and who is not running for president - particularly whether that enigmatic New Yorker, Mario Cuomo, is going to get into the race this time. The New York Times reports that the governor, who has enough economic problems at home to keep very busy, is making speeches in which he accuses the president of complacency in dealing with the recession and then proposes several federal tax changes that, he says, would aid recovery. Mr. Cuomo, in the view of Democratic strategists, would be their most effective candidate. He has the charisma and persuasiveness to give the Democrats their best chance of winning. GOP strategists feel the same way. "Beside Cuomo there really is no one else," GOP strategist Ed Rollins said when asked what Democrat could give Bush a run for his money in '92. Both Al Gore and Tom Harkin were mystery men, too, as they dropped by the same breakfast forum to talk of their possible presidential aspirations. They promised to inform us later, doubtless right after Labor Day, of their final decision about entering the race for president. They are attractive politicians. But nobody - certainly not the voters - seemed to care too much whether these senators deigned to run. Jay Rockefeller is another mystery candidate who may have given us a clue to his intentions when his wife, Sharon Percy Rockefeller, president and chief executive officer of WETA-TV in Washington, made some staff adjustments in that organization which, some observers say, would free her to campaign with her husband. As they say: Stay tuned.