Simplify the following text for a Grade 14 native English-speaking person.
Original: In 1977 , Rau became Chairman of the North Rhine-Westphalia SPD , and in 1978 Minister President of the state , were he remained until 1998 , with four successful elections for the SPD , which became strongest party in the Landtag each time and gained an absolute majority three times , in 1980 , 1985 , 1990 and finally 1995 .
Simplified: In 1978 Rau became Minister President of North Rhine-Westphalia , where he remained until 1998 , winning four elections for the SPD , which became biggest party in the Landtag each time and gained an absolute majority three times , in 1980 , 1985 , 1990 and finally 1995 .