Retiring Republican Sen. Thom Tillis (N.C.) ripped President Trump's threats against the North Atlantic Treaty Organization (NATO) in recent weeks, amid escalating tensions over the conflict in the Middle East.
"The president of the United States cannot withdraw from NATO," Tillis told ABC's Jonathan Karl in a Sunday interview on "This Week." "Now, having said that, the president can poison the well."
The president has ripped NATO allies over the countries' reluctance to join U.S.-Israel operations in Iran, referring to the alliance as "a paper tiger" and calling members "cowards" in recent days.
A central point of Trump's criticism revolves around efforts to reopen the Strait of Hormuz, the key waterway for oil exports that has effectively shuttered since the onset of the war late last month. He has argued that Europe and China rely more heavily on the trade route for oil than the U.S.
Several NATO allies, including the United Kingdom, Germany and France, signaled "a readiness to contribute to appropriate efforts to ensure safe passage through the Strait" on Thursday.
This criticism from Trump, who has long threatened to pull out of the organization, has sparked concern among some GOP lawmakers, including Tillis, who are warning the president against possibly leaving the alliance.
"The president can make it functionally defunct if he wants to," Tillis, a staunch defender of the alliance, said Sunday. "And you'd be hard pressed ... to find one [general who supports severing ties], because that has enormous, enormous risk in it."
"American lives have been saved by the NATO alliance, and American lives will be lost in great numbers without it," he continued.
Tillis, the GOP leader of the Senate's NATO group, on Sunday took issue with the president's characterization of NATO as "cowards," stating that member countries "don't appreciate the way they're being treated right now."
"I don't think that they're cowards," the GOP senator told Karl. "I think they're people that weren't consulted on a major military operation, and I'd have the same reaction if I was the head of state."
This is not the first time Tillis has expressed dismay over Trump's remarks toward the military alliance. The North Carolina Republican was an outspoken critic of the Trump administration's efforts to seize Greenland last year, warning about how the push could damage the U.S.-NATO relationship.