 Complete the following phrase:  Borisov was elected Mayor of Sofia in 2004. In December 2005, he was the founding chairman of the conservative political party Citizens for European Development of Bulgaria GERB, becoming its lead candidate in the 2009 general election. Borisov led GERB to a landslide victory in 2009, defeating the incumbent Socialist Party, and resigned as mayor of Sofia to be sworn in as Prime Minister. He resigned in 2013, after nationwide protests against the governments energy policy, but after leading GERB to victory in the 