From 2004 to January 2009 he was the leader and chairman, of a new Polish left-wing party called Social Democracy of Poland (SdPl), formed from a break-away group of SLD. He was a candidate in the presidential elections in 2005, but he got fourth place, with 10%.
Marek Borowski is an MP from Piła, but in the September 2005 parliamentary elections he contested a seat in Warsaw. Borowski was the Social Democratic presidential candidate in the 2005 Polish presidential elections. Just as his party received a massive defeat in the September 2005 Parliamentary elections, Borowski lost the presidential elections, receiving 10% of the vote and fourth place, despite Aleksander Kwaśniewski's support following the withdrawal of Włodzimierz Cimoszewicz.
He most recently ran (unsuccessfully) for the office of mayor of Warsaw in the 2010 local elections.