Donald Trump Jr. has pulled out of a scheduled appearance with George P. Bush after his father — former Florida Governor Jeb Bush — took a shot at President Donald Trump’s immigration policies.

The president’s eldest son will no longer help fundraise for Bush’s re-election campaign for Texas land commissioner on June 25 in New York.

Bush on Monday called on President Trump to end the “heartless” policy of separating immigrant minors from their parents at the border.

“Children shouldn’t be used as a negotiating tool,” the failed presidential candidate posted on Twitter. “@realDonaldTrump should end this heartless policy and Congress should get an immigration deal done that provides for asylum reform, border security and a path to citizenship for Dreamers.”

Children shouldn’t be used as a negotiating tool. @realDonaldTrump should end this heartless policy and Congress should get an immigration deal done that provides for asylum reform, border security and a path to citizenship for Dreamers. https://t.co/OOjv0vNeVg — Jeb Bush (@JebBush) June 18, 2018

The jab came in response to President Trump’s tweet vowing to stop “some of the worst criminals on earth” from exploiting minors to cross the border illegally.

“Children are being used by some of the worst criminals on earth as a means to enter our country,” Trump said. “Has anyone been looking at the Crime taking place south of the border. It is historic, with some countries the most dangerous places in the world. Not going to happen in the U.S.”

Attorney General Jeff Sessions announced in April a “zero-tolerance” policy towards illegal immigrants attempting to cross into the U.S. Over the past year, illegal immigrants smuggling children across the border posing as families grew by more than 300 percent.

At least 1,995 immigrant minors were separated from adults between April 19 and May 31, according to Department of Homeland Security officials. Once transferred to a border facility, the children are provided with food, shelter and medical care.