“This child separation policy is connected to the worst abuses of humanity in our history,” Mr. Schmidt said, reflecting the larger outpouring of anger over the administration’s zero-tolerance policy, which has included immigrants seeking asylum.

“It is connected by the same evil that separated families during slavery and dislocated tribes and broke up Native American families,” Mr. Schmidt said. “It is immoral and must be repudiated.”

He also accused several officials of being “complicit” in enabling the president’s policies, including Kirstjen Nielsen, the secretary of homeland security; Mitch McConnell, the Senate majority leader; and Speaker Paul D. Ryan.

Save for a few governors, Mr. Schmidt said, the Republican Party is now filled with “feckless cowards.” He said that the few who deserved to be spared that label included Govs. Charlie Baker of Massachusetts, Larry Hogan of Maryland and John Kasich of Ohio.

As a result, Mr. Schmidt called for a Democratic wave in the midterms, describing that party as the only one remaining “that stands for what is right and decent.”