Complex sentence: He was a leader of the right-wing element of the Labour Party, and an effective, if aggressive, election campaigner, but was ultimately unable to cope with the pressures of high office without excessive drinking.
Split into simpler sentences: He was a leader of the right-wing element of the Labour Party and an effective, if aggressive, election campaigner. Ultimately, however, he was unable to cope with the pressures of high office without excessive drinking.