WASHINGTON — A new conservative nonprofit group scored a $1.6 billion windfall last year via a little-known donor — an extraordinary sum that could give Republicans and their causes a huge financial boost ahead of the midterms, and for years to come. The source of the money was Barre Seid, an electronics manufacturing mogul, and the donation is among the largest — if not the largest — single contributions ever made to a politically focused nonprofit. The beneficiary is a new political group controlled by Leonard A. Leo, an activist who has used his connections to Republican donors and politicians...