It predates Truman. Earliest statistics I've seen were for 1916, Wilson (D) had 55%. The Jewish majority vote for Dem/Progressive/Socialist candidates grew from there. FDR got 90% as did LBJ (vs. Goldwater).
Reagan was the last Republican to make a significant dent getting 39% of the Jewish vote against Carter. The gains eroded and were gone by '92 when Bush 41 lost to Clinton.
Thanks for the info.