The coalition can be classified as left-wing, however, during the 1990s, it managed to attract voters from the pro-market and even right-wing camp. The main support for SLD came from middle-rank state sector employees, retired people, former Polish United Workers Party (PUWP) and OPZZ members and those who were unlikely to be frequent church-goers. The core of the coalition (Social Democracy of the Republic of Poland) rejected concepts such as lustration and de-communization, supported a parliamentarian regime with only the role of an arbiter for the president and criticized the right-wing camp for introduction of religious education into school. The excommunists criticized the economic reforms, pointing to the high social costs, without negating the reforms per se.
SdRP, SDU and some other socialist and social-democratic parties had formed the original Democratic Left Alliance as a centre-left coalition just prior to the nation's first free elections in 1991. In 1999 the coalition became a party, but lost some members.
At the time, the coalition's membership drew mostly from the former PUWP. An alliance between the SLD and the Polish People's Party (PSL) ruled Poland in the years 1993–1997. However the coalition lost power to the right-wing Solidarity Electoral Action in the 1997 election as the right-wing opposition was united this time and because of the decline of support for SLD's coalition partner PSL, though the SLD itself actually gained votes.
SLD formed a coalition with Labour Union before the 2001 Polish election and won it overwhelmingly at last by capturing about 5.3 million votes, 42% of the whole and won 200 of 460 seats in the Sejm and 75 of 100 in the Senate. After the elections, the coalition was joined by the Polish People's Party (PSL) in forming a government and Leszek Miller became the Prime Minister. In March 2003, the PSL left the coalition.
By 2004 the support for SLD in the polls had dropped from about 30% to just below 10%, and several high-ranking party members had been accused of taking part in high-profile political scandals by the mainstream press (most notably the Rywin affair: Rywin-gate).
On 6 March 2004, Leszek Miller resigned as party leader and was replaced by Krzysztof Janik. On March 26 the Sejm speaker Marek Borowski, together with other high-ranking SLD officials, announced the creation of a new centre-left party, the Social Democratic Party of Poland. On the next day, Leszek Miller announced he would step down as Prime Minister on 2 May 2004, the day after Poland joined the European Union. Miller proceeded to do so.
In the 2004 European Parliament election, it only received 9% of the votes, giving it 5 of 54 seats reserved for Poland in the European Parliament, as part of the Party of European Socialists. In the later 2009 European election the Democratic Left Alliance-Labor Union joint ticket received 12% of the vote and 7 MEPs were elected as part of the newly retitled Socialists & Democrats group.
Wojciech Olejniczak, the former Minister of Agriculture and Rural Development, was elected the president of SLD on 29 May 2004, succeeded Józef Oleksy, who resigned from the post of Polish Prime Minister due to alleged connections to the KGB.
However the SLD could not avoid suffering a huge defeat in the 2005 parliamentary election, in which it won only 11.3% of the vote. This gave the party 55 seats, barely a quarter of what it had had prior to the election. It also lost all of its Senators. In late 2006 a centre-left political alliance called Left and Democrats was created, comprising SLD and smaller centre-left parties, the Labour Union, the Social Democratic Party of Poland, and the liberal Democratic Party – demokraci.pl. The coalition won a disappointing 13% in the 2007 parliamentary election and was dissolved soon after in April 2008. On 31 May 2008, Olejniczak was replaced by Grzegorz Napieralski as SLD leader.
In the 2011 parliamentary election, SLD received 8.24% of the vote which gave it 27 seats in the Sejm. After the elections, one of the party members, Sławomir Kopyciński, decided to leave SLD and join Palikot's Movement. On December 10, 2011, Leszek Miller was chosen to return as the party leader.
In the 2014 European elections on 25 May 2014, the SLD received 9.4% of the national vote and returned 4 MEPs.
In July 2015 the SLD along with Your Movement (TR), Labour United (UP) and The Greens (PZ) formed the United Left electoral alliance to contest the upcoming election.
In the 2015 parliamentary election held on 25 October 2015, the United Left list received 7.6% of the vote, below the 8% threshold (electoral alliances must win at least 8% of the vote, as opposed to the 5% for individual parties), leaving the SLD without parliamentary representation for the first time. Indeed, no centre-left parties won any seats in this election.
SLD is often accused of Post-communism by right-wing political parties as large amount of SLD members were members of Polish United Workers' Party (PUWP) before 1990. Second argument given by the far-right is that SLD uses red color which is used by Marxist parties. SLD was founded by former secretaries of PUWP such as Aleksander Kwaśniewski and Włodzimierz Cimoszewicz.
The "Moscow Loan" was a loan of US$1.2 million by Communist Party of the Soviet Union to the PUWP in January 1990. The PUWP coordinators were Mieczysław Rakowski and Leszek Miller. After it was revealed in 1991, Włodzimierz Cimoszewicz advised Leszek Miller not to take Parliamentary oath, which according to the Polish constitution means automatic refusal to take Parliamentary seat, but Miller refused to do this and took his Parliamentary seat. On 29 May 1992, the Sejm gave permission for criminal procedure against Leszek Miller.