German Chancellor Angela Merkel to blame for letting US President Donald Trump humiliate her that way: Her rival

German Chancellor Angela Merkel is to blame for the way she has allowed US President Donald Trump to treat her, suggests her rival in the upcoming election.

Merkel's top opponent in the September election, Martin Schulz, said Monday that Trump's behavior towards the chancellor has been "unacceptable" and no country’s leader must "allow him or herself to be treated" that way, Deutsche Welle reported.

The US president delivered a condescending speech among NATO leaders Wednesday at the NATO headquarters in Brussels, where he failed to explicitly affirm Article 5 of the NATO charter, which stipulates allies to aid a fellow member under attack.

US President Donald Trump (R) delivers a speech as NATO heads of governments listen  during the unveiling ceremony of the new NATO headquarters in Brussels, on May 25, 2017, during a NATO (North Atlantic Treaty Organization) summit. (Photo by AFP)

He also censured 23 NATO members for not spending 2 percent of their GDP on defense.

Schulz, who was once the president of the European Parliament, voiced “outrage” over Trump’s treatment of Merkel, calling it “unacceptable.”

"Election campaign or no election campaign, in this situation let me be entirely clear: the chancellor represent all of us at summits like these," Schulz said. "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."

This is while Merkel said Berlin was still focused on boosting ties with Washington.

“[They] are a strong pillar of our foreign and security policy, and Germany will continue working to strengthen these relations,” Merkel’s spokesman, Steffen Seibert, told The Associated Press. “Precisely because they are so important, it’s right to name differences honestly."

After speaking with Trump at the recent Group of 7 (G7) summit, Merkel had said Germany could no longer "completely depend" on the US as a partner.

Trump has not been a great fan of the North Atlantic Treaty Organization like most other US alliances with Western partners.