Complex sentence: Early in his career, he was a "loyalist" supporter of King and Country; later, he joined (and successfully publicised) the radical movement which led to the Reform Bill of 1832 and him winning the parliamentary seat of Oldham.
Split into simpler sentences: Early in his career, he was a loyalist supporter of King and Country. Later, he joined and successfully publicised the radical movement which led to the Reform Bill of 1832, and to his winning the parliamentary seat of Oldham.