The Conservative Party and Labour Party have been the two biggest political parties, and have supplied every Prime Minister since 1922. The Conservative Party have governed since the 2010 election, in coalition with the Liberal Democrats from 2010 to 2015. At the 2015 general election the Conservative Party committed to offering a referendum on whether the UK should leave the European Union and won a majority in that election. A referendum was held in June 2016, and the Leave campaign won by 51.9% to 48.1%. The UK initiated the withdrawal process in March 2017, and Prime Minister Theresa May triggered a snap general election in 2017, in order to demonstrate support for her planned negotiation of Brexit. The Conservative Party lost seats – they won a plurality of MPs, but not a majority. As a result, they formed a minority government, with the Democratic Unionist Party (DUP) as their confidence and supply partner. Neither May nor her successor Boris Johnson (winner of the 2019 Conservative Party leadership election) was able to secure parliamentary support either for a deal on the terms of the UK's exit from the EU, or for exiting the EU without an agreed deal. Johnson later succeeded in bringing his Withdrawal Agreement to a second reading in Parliament, following another extension until January 2020.