French presidential hopeful Manuel Valls has pledged to tax US imports if Donald Trump pulls America out of the global climate deal reached in Paris in 2015 and declares "economic war on Europe".
Mr Valls, who seeks the Socialist nomination, proposed a carbon tax on US goods while speaking on RTL radio.
He said: "When Donald Trump declares some kind of economic war on Europe, we must be ready."
Mr Valls, a former prime minister, is facing ex-government member Benoit Hamon in the country's Socialist presidential primary run-off on Sunday.
Mr Hamon said environmental and climate issues are among his priorities.
During his campaign, US president Mr Trump suggested he was keeping an open mind about climate change and about whether he will pull America out of the global emissions-cutting deal struck in Paris.