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The head of Germany's center-left Social Democrats is lobbying party members to vote for opening coalition talks with Chancellor Angela Merkel's conservatives amid strong opposition from grassroots members.

Martin Schulz was in Dortmund Tuesday to push for approval of the formal negotiations with the Union bloc at a party convention Sunday.

A rejection of talks would be a setback for both Schulz and Merkel , who has already failed to forge a coalition with two smaller parties.

The Social Democrats and the Union bloc, who have governed Germany in a "grand coalition" since 2013, suffered heavy losses in September's national election.

In non-binding votes, Social Democrats in the smaller states of Berlin and Saxony-Anhalt have indicated they'll vote against opening coalition talks, while party members in Brandenburg voted in favor.