Complex sentence: In 1869 Rublee was a candidate for the US Senate, although this was prior to the Seventeenth Amendment (the direct election of Senators), so while the candidacy and campaign were not a secret from the public, was carried out almost entirely behind closed doors.
Split into simpler sentences: In 1869 Rublee was a candidate for the US Senate, although this was prior to the Seventeenth Amendment according to which voters became empowered to directly elect US Senators. The candidacy and campaign while not a secret from the public, was carried out almost entirely behind closed doors.