Complex sentence: Roosevelt's most formidable challengers were his former campaign manager James Farley and his Vice President, John Nance Garner, who had also sought the nomination for the presidency and soundly lost to Roosevelt who would be "drafted" at the convention.
Split into simpler sentences: Roosevelt's most formidable challengers were his former campaign manager James Farley and his Vice President, John Nance Garner. Both had sought the nomination for the presidency and soundly lost to Roosevelt who would be "drafted" at the convention.