and successfully passed several economic reforms. He was eventually rewarded in December 1999 with an invitation to begin membership talks with the European Union. But public discontent over the social cost of the reforms, including increased unemployment, as well as allegations of corruption led to the SDS's defeat in the June 2001 elections, which were won by the National Movement for Simeon II. The United Democratic Forces won 18.2% of the popular vote and 51 out of 240 seats. The SDS was chaired until the May 20, 2007 European elections by Petar Stoyanov, former president of the country. Meanwhile, Kostov, the former Prime Minister and SDS party leader went on to form his own party - Democrats for a Strong Bulgaria. At the 2005 parliamentary election, the United Democratic Forces won 8.4% of the popular vote and 20 out of 240 seats. It was announced at the inaugural conference of the Movement for European Reform (MER, March 2007) that the SDS would become official partners alongside the British Conservative Party and the Czech Civic Democratic Party. In mid April 2007, the SDS backtracked on its decision, stating that it remains loyal to the EPP and that it will never leave the EPP section of the EPP-ED Group to join another Group. In the May 20, 2007 European elections the SDS failed to elect even a single MEP, resulting in the resignation of Stoyanov who led the list. At the start of 2009 the SDS entered in an electoral alliance named the "Blue Coalition" with four other center-right parties: the DSB, the United Agrarians, the Bulgarian Social Democratic Party and the Radical Democratic Party. The alliance fielded candidates for the June 2009 European election winning a seat. When the Lisbon Treaty came into force in December of that year, their representation doubled. Now both the SDS and the DSB had an MEP. The Blue coalition placed fifth in the legislative election a month later with a combined 6.8% of the vote and 15 seats. In the May 2013 elections, the SDS ran alone and lost all of its seats in the Bulgarian National Assembly, but regained four next year as a part of the Reformist Bloc alliance. The same pattern repeated in 2017 and April of 2021, with zero and then two seats respectively (running together with GERB in 2021). Two other MPs joined the SDS later on. Coalitions United Democratic