Complex sentence: A member of the Democratic Party, Leahy served as the President pro tempore of the United States Senate from December 17,2012 to January 6,2015; as President pro tempore, he was third in the presidential line of succession.
Split into simpler sentences: A member of the Democratic Party, Leahy served as the President pro tempore of the United States Senate from December 17,2012 to January 6,2015. As President pro tempore, he was third in the presidential line of succession.