The SDPL should not be confused with a former party the Social Democracy of the Republic of Poland (SdRP) which existed between 1990–99 and was a direct predecessor of the SLD.

In May 2005 the party reached an agreement with Labour Union (UP) and Greens 2004 to jointly contest for 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.

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).