Boris Yeltsin's condition after a heart attack appears more serious than his physicians had at first let on and has assumed political dimensions. Moscow bureau chief John Kohan reports that although the 64-year-old Russian President's heart problem does not appear to be life-threatening, his condition seems more serious than during his last heart attack in July. One worrisome sign: no one was allowed in to see Yeltsin today but doctors and his security chief, Alexander Korzakhov. Doctors now say Yeltsin must remain under close medical supervision for a month. The problem has already forced the cancellation of next week's scheduled Moscow summit with the presidents of Bosnia, Croatia and Serbia. If Yeltsin recovers as expected, his political opponents can be expected to exploit his ill health in the campaign for the presidential election next June.