The dramatic announcement came after contentious meetings with Trump, who had used his first official trip to Europe to criticize German trade, scold world leaders about their NATO spending and refuse to commit to the Paris agreement on combating climate change.

Schulz, a former president of the European Parliament, is the head of the center-left Social Democrats. He is easily the most convincing challenger to Merkel's 11-year reign as chancellor and a charismatic leader in his own right. Yet rather than criticize his rival or her Christian Democratic Union-led government for the strained relationship with Trump, Schulz has passionately offered support.

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In video published by German broadcaster Deutsche Welle on Monday, a visibly angry Schulz can be seen railing against Trump, who he said “believed he could inflict humiliation in Brussels.”

The Social Democrat leader then said it did not matter that Merkel and he were in the middle of an election campaign, as “the chancellor represents all of us at summits like these, and I reject with outrage the way this man takes it upon himself to treat the head of our country's government.”

“That is unacceptable,” Schulz said.

He had made a similar comment on Sunday shortly after Merkel's remarks. “A stronger cooperation of European countries on all levels is the answer to Donald Trump,” Schulz told the public broadcaster ARD. On Monday, he tweeted in German, English and French that the “best response to Donald Trump is a stronger Europe.”

Such comments suggest that Trump's reputation in Europe may end up influencing the continent's politics in unexpected ways. Although the possibility of a Schulz chancellery has become less and less likely after poor showings by the Social Democrats in recent regional elections, the center-left's animosity toward Trump is broadly shared by many German voters.

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And Trump doubled down on his criticism Tuesday, calling out Germany's trade and defense policies as “very bad” for the United States. “We have a MASSIVE trade deficit with Germany, plus they pay FAR LESS than they should on NATO & military,” Trump wrote in a Twitter post. “Very bad for U.S. This will change.”

In a poll conducted in February, 78 percent of Germans said they were “very concerned” about Trump's policies — almost 20 percent more than those who were worried about the politics of Russian President Vladimir Putin.

Characteristically, Schulz has taken a more direct tone in his criticism than the soft-spoken Merkel, calling Trump's policies “un-American” in late January. Notably, he has pushed back on long-standing U.S. demands for Germany to spend 2 percent of its gross domestic product on defense — something Trump pushed during last week's meeting of the leaders of the North Atlantic Treaty Organization.

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However, while the Christian Democrats were beneficiaries of a short-lived “Trump bump” after the 2016 U.S. presidential election, Merkel also may have found ways to use Germans' animosity toward Trump to her advantage. Thanks to a quirk of scheduling, before she met Trump in Brussels last week, she spent a morning with former U.S. president Barack Obama, who is still broadly admired in Germany.

After Merkel's comments about Trump at the beer hall, she received applause for a whole minute. The next day, her spokesman reiterated her intent. “The chancellor's words stand on their own,” Steffen Seibert said during a news conference Monday. “They were clear and comprehensible.”

Brian Murphy contributed to this report.