Withdrawal was advocated by hard Eurosceptics and opposed by pro-Europeanists and soft Eurosceptics, with both sides of the argument spanning the political spectrum. The UK joined the European Communities (EC) – principally the European Economic Community (EEC) – in 1973, and its continued membership was endorsed in the 1975 referendum. In the 1970s and 1980s, withdrawal from the EC was advocated mainly by the political left, e.g. in the Labour Party's 1983 election manifesto. The 1992 Maastricht Treaty founded the EU, was ratified by the British Parliament in 1993 but was not put to a referendum. The Eurosceptic wing of the Conservative Party led a rebellion over ratification of the treaty and, with the UK Independence Party (UKIP) and the cross-party People's Pledge campaign then led a collective campaign particularly after the Treaty of Lisbon was also ratified by the European Union (Amendment) Act 2008 without being put to a referendum following a previous promise to hold a referendum on ratifying the abandoned European Constitution was never held eventually pressured the Conservative prime minister David Cameron to hold the June 2016 membership referendum. Cameron, who had campaigned to remain, resigned after the result and was succeeded by Theresa May.