Complex sentence: More importantly, they were only 10 seats behind the Liberals--the closest before or since the party and its predecessors had ever gotten to the two major parties up to that point, and the best performance for a third party in almost 60 years.
Split into simpler sentences: More importantly, they were only 10 seats behind the Liberals--the closest the party and its predecessors had ever gotten to the two major parties up to that point. It was also the best performance for a third party in almost 60 years.