“For too long,” Mr. Reid is expected to say, according to prepared remarks, “Senator McConnell and Speaker Ryan have tried to have it both ways, giving Trump a slap on the wrist each time he says something detestable, but always committing to support him at the end of the day.”

“This is precisely the kind of moral cowardice that enabled the rise of Trump,” Mr. Reid is expected to add. “After years of refusing to denounce birtherism, McConnell and Ryan are now supporting the most prominent birther in the nation.”

“Why are they waiting to withdraw their support? What more do they need to see?” he is expected to ask.