Martin Schulz | Steffi Loos/Getty Images Schulz to Trump: Dropping Paris agreement means no trade talks ‘Whoever wants to have access to our market … needs to respect the European standards,’ Schulz says.

Germany's challenger for the chancellorship, Martin Schulz, vowed Thursday to retaliate against U.S. President Donald Trump's potential withdrawal from the Paris climate agreement by refusing to engage in transatlantic trade talks.

Referring to trade negotiations with the U.S., which ran until the end of last year but are now on ice, Schulz said it would be impossible to grant better market access to the U.S. if it did not respect climate protection rules.

"If the U.S. drops out of the climate agreement ... for European trade policy, this means that American production sites don't need to abide by the climate goals," said the Social Democratic candidate, who was speaking at the WDR Europa Forum in Berlin. "That is a competitive distortion against which we can only protect ourselves by saying: Whoever wants to have access to our market, and the European market is the biggest market in the world, needs to respect the European standards."

Schulz, who hopes to beat Chancellor Angela Merkel in federal elections in September, said this logic of ensuring the enforcement of European standards was the reason why he supported free trade agreements in general.

U.S. Commerce Secretary Wilbur Ross on Tuesday indicated interest in resuming talks on a transatlantic trade pact. Ross said it "makes sense ... to work towards a solution that increases overall trade while reducing our trade deficit.”