The president has been involved in a tit-for-tat bickering marathon with key GOP Senators John McCain, Bob Corker and Jeff Flake.p Experts said the spats with his own party members threaten to derail Trump’s legislative agenda, as Republicans have a very slim majority and need every vote they can muster.

GOP senators’ disagreements with Trump “are ominous because Republicans have a very narrow margin in the U.S. Senate and can’t afford to lose more than a couple of votes,” Brookings Institution Senior Fellow Darrell West told Xinhua.

On Tuesday, Trump mocked Corker by stating on Twitter that the senator “couldn’t get elected dogcatcher,” referring to a job in which a local government employee would catch stray canines and place them in a shelter.

That was followed the same day by a scathing tweet by Corker, who referred to the White House as “an adult day care center,” and accused Trump of being “untruthful”.

On Wednesday, Trump hit back, tweeting that “The reason Flake and Corker dropped out of the Senate race is very simple, they had zero chance of being elected.”

Over the weekend, U.S. cable news TV station C-SPAN interviewed McCain, who had spent several years as a prisoner of war in Vietnam, about the Vietnam War. The senator took a thinly veiled swipe at Trump.

“One aspect of the conflict, by the way, that I will never, ever countenance is that we drafted the lowest income level of America,” said the veteran. “And the highest income level found a doctor that would say they had a bone spur. That is wrong. That is wrong. If we are going to ask every American to serve, every American should serve.”

The comment was a direct jab at the president. While the senator did not mention Trump by name, Trump has in the past said he was excused from military service during the Vietnam War due to bone spurs — a condition of the bones.

During the Vietnam War era, many people received deferments from military service due to medical conditions. Americans’ popular memory of the era is that young men with the right family connections could pull the right strings and escape the military draft for a number of reasons. That left lower income young men to serve on the front lines, although some dispute this as not fully accurate.

The ill will between Trump and McCain started when Trump blasted McCain for not voting in favor of the president’s major healthcare bill, a repeal of the previous administration’s controversial healthcare law called Obamacare. That left Trump short of the votes he needed to pass the bill, and the legislation failed to pass.

The spat has continued for a number of weeks, and experts said it could damage the White House’s agenda, as well as Trump’s decision to make key decisions.

“McCain … has the potential to vote against Trump on (future) issues. That would make it nearly impossible for Trump to implement his policy agenda and alter the course of U.S. affairs,” West said.

“Trump should take this seriously because McCain can combine with a couple of other Republican Senators and derail Trump’s legislative agenda. That would deal a serious blow to the Trump Administration,” West said.

Dan Mahaffee, senior vice president and director of policy at the Center for the Study of Congress and the Presidency, also told Xinhua that these spats continue because Trump is dealing with a sense of embattlement, an inclination to favor his base over other constituencies, pressure to make achievements before the 2018 Congressional elections, and rifts in the GOP that have been widening for decades.

This disunity, alongside the rumored rifts in the cabinet and the White House staff, present an image of a fractured government in dealing with both the domestic policy challenges, as well as a wide range of diplomatic and national security challenges.

“It is hardly reassuring for American allies and a wide range of the American people,” Mahaffee said.