BERLIN (AP) — Martin Schulz, the center-left challenger running against Chancellor Angela Merkel, has vowed to get his Social Democratic party into power at Germany's national election in September.

Schulz, speaking at a party congress Sunday in Dortmund, focused on social justice. He sharply condemned the country's populist AfD party, lobbied for better integration of immigrants and promised that his party won't sign any coalition deals with other parties unless they back the right to gay marriage in Germany.

Polls currently give Merkel's conservatives a double-digit national lead ahead of the Sept. 24 vote, with Schulz's Social Democrats sagging following a surge earlier this year. Both parties hope to escape the "grand coalition" in which they now govern Germany together.

The SPD party congress will formally approve the party program later Sunday.