WASHINGTON — A top House Republican’s little-known past job working with Democrats to change US elections and choose presidents by popular vote threatens to upend his bid to rise in the GOP ranks, The Post has learned. The three-way House Republican whip race between Reps. Tom Emmer (R-Minn.), Jim Banks (R-Ind.) and Drew Ferguson (R-Ga.) is expected to be the only contested GOP leadership vote and will be settled by secret ballot about a week after the Nov. 8 midterm elections, which Republicans are favored to win. Emmer, who is regarded as the most moderate option, is overseeing House GOP...