The Ontario New Democratic Party is a social-democratic political party in Ontario, Canada. The Ontario NDP, led by Andrea Horwath since March 2009, currently forms the Official Opposition in Ontario following the 2018 general election. It is a provincial section of the federal New Democratic Party. It was formed in October 1961 from the Co-operative Commonwealth Federation and the Ontario Federation of Labour. For many years, the Ontario NDP was the most successful provincial NDP branch outside the national party's western heartland. It had its first breakthrough under its first leader, Donald C. MacDonald in the 1967 provincial election, when the party elected 20 Members of Provincial Parliament to the Ontario Legislative Assembly. After the 1970 leadership convention, Stephen Lewis became leader, and guided the party to Official Opposition status in 1975, the first time since the Ontario CCF did it twice in the 1940s.