David Leonhardt: “If Republicans do manage to keep both chambers of Congress, it would cause a political shock. So far, much of the speculation has focused on what a Democratic House takeover might mean — attempts to rein in Trump’s executive actions, subpoenas, investigations, maybe even impeachment. But it’s important to understand that a Republican victory would also change Washington.” “It would be validation for Trump, who could then brag that he had defied the experts once again.