Simplify the following text for a Grade 6 native English-speaking person.
Original: Conceding defeat in the September 2006 general election , he announced that he would resign as party leader , and Mona Sahlin was elected to succeed him as party leader in March 2007 .
Simplified: He was the leader of the Social Democratic political party , but quit in March 2007 . He was replaced by Mona Sahlin as leader of the party , the partys first woman to do so .