MIKHAIL GORBACHEV may be unpopular with his people and losing his grip on power as the Soviet Union fractures. But this week's summit meeting in Moscow with President Bush will give Mr. Gorbachev a boost.As usual, this boost comes just as Gorbachev needs it. He skillfully sandwiched the Communist Party plenum between his visit to London for the Group of Seven meeting and the summit with President Bush Wednesday. During the plenum, he stepped back into the political mudpit of the party - which he still leads. The future of the Soviet Union and, Gorbachev realizes, his own future are not with the old Communist Party. Gorbachev's strategy seems to be to isolate the party in whatever way he can - such as declaring an end to Leninist ideology and making noises about alternative parties. The party will only be an impediment to the "nine-plus-one" Union Treaty between the Soviet center and the republics. Still, Gorbachev joined others at the plenum in decrying the move by Boris Yeltsin, president of the Russian republic, to oust Communist cells from factories and state offices. Increasingly, the future of the Soviet Union appears to be in some form of the Union Treaty confederation. The Soviet center would provide basic military and diplomatic needs, and communications. The republics would develop their own economic deals and political autonomy - as they are currently doing anyway. Despite heated speculation last week that Gorbachev might leave the party, he could not. Not yet. His departure would only consolidate a conservative hard-line bloc against him. The hard-liners are making trouble enough for Gorbachev as it is. He can't afford simply to hand them the party's wealth and its apparatus. Better to stay and keep them as much in line as possible. The Union Treaty will bring a major restructuring of Soviet authority. Until it happens, the old military and police relationships must be watched over.