Ohio Gov. John Kasich (R) during an appearance in 2015. (Steve Helber/AP)

Ohio Gov. John Kasich, who’s weighing a Republican primary challenge against President Trump in 2020, on Thursday criticized his “wrecking ball” diplomacy shortly after Trump departed a tumultuous NATO summit in Brussels.

“Wrecking ball strategies rarely end up being successful over the long term,” Kasich said during an appearance in Washington, where he also questioned the wisdom of Trump’s trade war with China and imposition of tariffs on other trading partners.

“We’ve been bullying countries all over the world,” the governor said at the outset of his remarks at the National Press Club.

Kasich, who competed unsuccessfully against Trump for the GOP nomination in 2016, insisted that his appearance was not intended to be political and said he remains undecided about his future. He said he was speaking out because it was important for another politician to air concerns about Trump’s policies.

“This is not a happy day for me,” Kasich said. “I wish I wasn’t here, to tell you the truth.”

His remarks followed a two-day NATO summit at which Trump claimed to have successfully pressured other members to step up their defense spending but at which his brusque style rankled U.S. allies.

“The alliance that has kept the peace for 70 years is being frayed,” Kasich said. “The fraying of relationships has consequences. . . . If this alliance begins to fray in a more serious way, what takes its place?”

Kasich said that he supports Trump standing up to China on trade issues but that the tariffs Trump has imposed are not the right strategy.

Kasich also praised the Republican-led Senate for passing nonbinding measures in recent days calling for a greater role in overseeing Trump’s trade decisions and voicing support for NATO.