The SLD-UP won a stunning victory at the 2001 parliamentary elections, gaining 41% of the vote, and formed the government with the Polish People's Party. During the 2001–2005 term, there occurred a splintering of the alliance and there came into being four left-wing political caucuses in the Polish parliament:
The SLD-UP maintained their alliance for the European parliamentary elections of 2004, and won five seats with 9.3% of the vote, a sharp decline from the national elections in 2001.
Following the relatively poor performance of the SDPL-UP-Greens at the 2005 parliamentary elections (they polled 3.9% and failed to break the 5% parliamentary threshold), and the overwhelming success of right-wing parties, the SDPL and UP sought to reapproach the SLD, and seek a new understanding. A new alliance was born in 2006-07, the Left and Democrats, which consisted of the three left-wing parties, SLD, SDPL and UP as well as the centrist Democratic Party (PD). Left and Democrats contested the 2007 parliamentary elections and polled 13.2% of the vote. SLD, SDPL and PD saw several of their representatives elected to the Sejm, but the UP failed to elect any members. In 2008, following a rift between the constituent parties of the Left and Democrats, the alliance came to an end with PD and SDPL leaving the alliance in April. These two went on to form the Agreement for the Future, together with the Greens 2004, for the 2009 European elections. The Labour Union, however, chose to maintain its alliance with SLD, and at the elections SLD-UP polled 12.3%, electing seven representatives to the European Parliament.
The SLD-UP coalition was reactivated for the 2014 European Parliament elections. It received 9.4% of the votes and won five seats.
