Complex sentence: Since then, he has enjoyed unchallenged domination in Suphanburi; in every election he has participated, he has won by a landslide victory, receiving 60--90 per cent of the votes cast.
Split into simpler sentences: Since then, he has enjoyed unchallenged domination in Suphanburi. In every election he participated, he won by a landslide victory, receiving on average 60 to 90 per cent of the votes cast.