President Trump and German Chancellor Angela Merkel pose for a photo before a bilateral meeting on the eve of the G-20 summit in Hamburg on July 6. (Pool photo by Michael Kappeler via AP)

BERLIN — President Trump's domestic approval rating stands at an abysmal 36 percent, but in Europe, where he is even more deeply unpopular, the U.S. president is just about the most reliable political punching bag for politicians striking from the left.

Fresh evidence of that came Monday, when Martin Schulz, leader of Germany's Social Democratic Party and the man vying to unseat Chancellor Angela Merkel, inserted himself into the latest scandal plaguing Trump's administration — Donald Trump Jr.'s meeting last summer with a Russian attorney said to have “dirt” on Hillary Clinton.

Trump has mounted a full-throated defense of his son. He wrote Monday morning on Twitter that “most politicians” would have attended such a meeting. “That's politics!” the president added.

Schulz replied in English to Trump's Tweet, writing: “I wouldn't have gone there. This is not politics.”

The candidate's willingness to chastise the sitting U.S. president reflects the depths of anti-Trump sentiment in Germany. But it's also evidence of a no-holds-barred approach by a politician struggling to catch on domestically.

After igniting a groundswell of support for the center-left Social Democrats when he was named their candidate at the beginning of the year, Schulz has fallen steeply in the polls. He now trails Merkel by nearly 30 points, according to recent polling.

In an effort to reboot his struggling campaign, the former head of the European Parliament has made Trump a regular feature of his stump speeches. He rails against Trump's decision to withdraw the U.S. from the Paris climate agreement and attacks Merkel for countenancing his demand that European countries spend more on defense.

Speaking at an event in Berlin in May, Schulz vowed that he would resist Trump.

“It is the order of the day that one puts obstacles in the way of that man,” he said.

His predecessor, Sigmar Gabriel, has voiced similarly harsh criticism, calling Trump the “front-runner of a new authoritarian and chauvinist movement." Gabriel described Trump’s inauguration speech in January as “highly nationalistic.”

The strategy is based on sound numbers, whether it is paying off: 87 percent of Germans lack confidence in Trump, according to Pew data.

Merkel, meanwhile, has walked a tightrope. She has emphasized the resilience of German-American ties and the range of interests that join the two countries. But she has also made no secret of her disagreement with the American president, particularly when it comes to trade and the environment.

Schulz's rejoinder to Trump on Twitter may have resonated not just with Germans but with Americans, too. Most found the meeting with the Russian attorney — attended by Trump's son, his son-in-law and several others — inappropriate, according to a new Post-ABC News poll.

Or it may not have. Most Republicans said the meeting was appropriate, the poll found, and people's perceptions were strongly skewed by their existing impressions of the president.

Rick Noack and Alexandra Rojkov contributed to this report.