Complex sentence: Norris left the GOP in 1936 (since seniority in the minority party was useless, and the Democrats offered him chairmanships) and was re-elected to the Senate as an Independent with some Democratic Party support in 1936.
Split into simpler sentences: Norris left the Republicans in 1936 since seniority in the minority party was useless, and the Democrats offered him chairmanships. He was re-elected to the Senate as an Independent with some Democratic Party support in 1936.