The plight of Jews in the Soviet Union has continued to worsen during the six months since Mikhail S. Gorbachev took over the Kremlin leadership, American Jewish groups told Congress. Morris B. Abram, chairman of the National Conference on Soviet Jewry, said the situation has declined in recent months with a “virtual halt in emigration” and “heightened intimidation” and harassment. “The situation . . . is more difficult than in any time in recent memory,” Morey Schapira, president of the Union of Councils for Soviet Jews, added in written testimony at a hearing of a House Foreign Affairs subcommittee.