There may be one last favor the Kremlin can do for George Bush before he faces the disgruntled American voters on Nov. 3. Indeed, some of his campaign staff are hoping against hope for some kind of miracle. A brisk coup in Moscow, in which Boris Yeltsin is replaced by a group of hard-line ex-Communists and military men who announce that they will not be dismantling those SS-18 heavy missiles, might just save the day for Bush's presidency. It sounds unlikely. It sounds downright dangerous for the world, and for Europe, and for whichever president occupies the White House after Nov. 3. It also sounds deeply unpleasant for Yeltsin and the Russian people. But it is a measure of the desperation felt by the Bush-Quayle campaign that they now count on a miracle to avert what the opinion polls say will be the biggest electoral landslide for the Democrats since Lyndon Johnson swamped the arch-conservative Barry Goldwater back in 1964.