The Republicans (some reluctantly) embraced Trump as a useful idiot who would sign the legislation they expected a Republican congress to grind out. They underestimated not only his corruption, incompetence and instability, but also their own. Paul Ryan and Mitch McConnell are easily the two biggest hypocrites and cowards in Washington, and perhaps in history (time will tell). Ryan and McConnell have a laser-like focus on one thing: taking health care away from as many people as they can in order to transfer the money to the wealthiest people in the country. They will abandon Trump when and if they determine that he is no longer a useful idiot but merely an ordinary idiot who impedes their pursuit of tax cuts for the wealthy.
Trump could have terminated Comey any time before Comey announced the investigation of the Trump campaign. After that, the termination is what it appears to be: a criminal attempt to cover up a federal investigation. History will see it as a turning point.