Complex sentence: They won 38 seats and became the party with majority in Assembly, but due to a dubious law meant to reduce the power of Ministers and the Cabinet, they decided to remain as opposition party instead of forming the government.
Split into simpler sentences: They won 38 seats and became the party with majority in Assembly. But due to a dubious law meant to reduce the power of Ministers and the Cabinet, they decided to remain as opposition party instead of forming the government.