BERLIN (AP) — The man hoping to beat Chancellor Angela Merkel in Germany's general election says his center-left Social Democrats have to be prepared for a tough campaign, after suffering a heavy blow in their traditional heartland.

Martin Schulz said Monday that "until the general election on Sept. 24 we have a long and stony road ahead of us."

Schulz's Social Democrats lost Germany's most populous state North Rhine-Westphalia to Merkel's Christian Democratic Union on Sunday.

Pressed on the issues he hoped to win over voters with in four months, the former European Parliament president indicated that his party would take a hard line in negotiations over Britain's departure from the EU.

National polls show the Social Democrats trailing Merkel's conservatives by up to 10 points after drawing level earlier this year.