A. Redistricting. The Fourth District lost SE areas of Queens (Springfield Gardens, Cambria Heights, Rosedale, etc.), which at the time were in considerable racial transition (from Jewish/Italian/German to black). After 1970, it became an all-Nassau district. All the Dem votes in the Five Towns couldn't hold up to the pubbie votes in Garden City, Franklin Square, Rockville Centre, etc.
B. The backlash against bussing and Lowenstein's antiwar stance.
Even prior to redistricting, Lowenstein barely won the retiring ‘64 LBJ baby Herbert Tenzer’s seat in ‘68 against Republican Mason Hampton. From ‘53-’65, this had been the seat of Republican Frank Becker. Becker’s grandson would later run unsuccessfully against McCarthy, but he didn’t have enough $$ support.