Jonathan Bernstein: Trump runs the Republican Party. Until it runs him.

Less than two weeks ago, President Donald Trump came up with a name for congressional Republicans who wouldn’t support higher age limits for any gun purchases: cowards. Now he’s backed away entirely from that position. This is more or less par for the course for Trump, although it’s a particularly stark example of how he undercuts his own allies.Does that kind of behavior harm him?It’s true that congressional Republicans have treated him pretty well. There’s a serio