"With regard to the future, we exclude a coalition with The Left at national level and with good reason," said the party's vice-president, Foreign Minister Frank-Walter Steinmeier, referring to a bloc that includes former communists.
Steinmeier also warned party leaders in the western state of Hesse to think carefully before entering into a pact with the extreme-left grouping in the regional parliament.
"The freedom to decide also carries with it a responsibility not to do something that can hurt our party in the rest of the country," he said.
The remarks were made at a party conference where the Social Democrats (SPD) conducted a post-mortem of three regional elections in the past two months that are seen as pointers to the national vote in 2009.
The party is seen as weakened by national president Kurt Beck's decision to approve a working agreement in Hesse with The Left, which groups members of the defunct East German communist party and Social Democrat defectors.
It earned Beck a tongue-lashing from Germany's conservative Chancellor Angela Merkel, who has governed in an increasingly fractious left-right national coalition with the Social Democrats since 2005.
Merkel said the SPD, and notably Beck, "are not reliable" and hinted that she was shopping around for other possible partners for the post-2009 period.
The Social Democrats registered gains in the regional votes in Hesse and in Hamburg in the north but are seen as being badly weakened by infighting over cooperation with The Left.
Der Spiegel magazine reported in its latest issue that senior party members want Steinmeier to replace Beck ahead of the 2009 vote.
But Steinmeier dismissed suggestions that the SPD is in dire straits and plans to ditch Beck, telling reporters: "We are not in distress, there is no reason for that."
He said the party on Monday voted overwhelmingly to preserve the right of regional leaders to take independent decisions over coalition arrangements, describing the fact as "a clear signal of support for Kurt Beck."
The recent regional elections confirmed that The Left is a political force on the rise.
It now holds seats in 10 of Germany's 16 regional legislatures but is seen an ideological outcast by other parties, though it offers a fresh option out of the post-election deadlock that is common in Germany.
Merkel vowed last week that her Christian Democrats would shun the party.