SDPL contested its first elections in June 2004, these being for Polish representation to the European parliament. The party gained 5.3%, which saw 3 members elected to parliament. In May 2005 the party reached an agreement with Labour Union (UP) and Greens 2004 to jointly contest the forthcoming Polish parliamentary elections, under the SDPL banner. SDPL managed to gain 3.9% of the vote, but fell short of the 5% threshold required to win parliamentary representation. SDPL put forward its party leader Marek Borowski, as candidate for the Polish presidential elections held in the following month of October. Borowski came fourth in the first round, winning 10.3% of the vote.
On September 3rd, 2006, SDPL joined the newly formed Left and Democrats (LiD) coalition, made up of four centre and centre left parties: SDPL, SLD, UP and Democratic Party – demokraci.pl. This alliance was created with a view to jointly contest the upcoming local government elections. The LiD alliance was maintained for the Polish parliamentary elections of October 2007, and LiD achieved 13.2% of the vote. This translated into 53 lower house seats, 10 of which were won by SDPL.
After LiD dissolved, 8 out of 10 SDPL MPs formed a new parliamentary caucus called Social Democracy of Poland - New Left (Socjaldemoracja Polska - Nowa Lewica, SDPL-NL).