Gingrich is part right, and part wrong, and it gets at what is at stake, in terms of oversight, in 2018.
Congress, legally and constitutionally, is supposed to oversee the executive branch. The president is supposed to carry out Congress's instructions, take care that the law is faithfully executed, blah, blah, blah. How do we ensure this? Congress brings members of the executive branch to give testimony and file documents, demonstrating that the law is being followed. It is part of the whole checks-and-balances thing. Right now, there is precisely zero oversight going on, as it relates to Donald Trump. The Republican majorities in both chambers are running cover-up operations, political hit-jobs on Mueller, and basically trying to make it as easy as possible for the President to commit crimes out of naked partisan ambition.
So, what happens if the House flips? The Senate is pretty much GOP, given the map, but for oversight purposes, a party just needs one chamber. Why? Because majority status determines who chairs the committees, and they are the ones with the authority to issue subpoenas, mostly. It's more complicated than that, there are votes involved, but committee chairs are the critical actors here. With majority status in one chamber you can issue subpoenas. Whether that is for Trump's tax returns, business records involving Russia or Saudi Arabia, or whatever other sleazy shit you want to investigate, a committee chair can bring executive branch employees in for testimony, and demand documents. Also, those tax returns.
But, Gingrich is part right, and part wrong. The right part is that Democrats will try. The right part is that Kavanaugh will also try to shield Trump, and the rest of the conservatives are likely to be sympathetic to him. But...
What if Roberts had an NFIB v. Sebelius moment? Do you seriously think Trump would hand over his tax returns? If the demand was to compel testimony from some executive branch employee, do you seriously think that person wouldn't claim executive privilege? Executive privilege is the principle by which executive branch employees, primarily cabinet secretaries and White House staff can decline to testify on the grounds that they must be able to provide the president with candid advice, which they cannot do if they can later be compelled to reveal what they said. Do you seriously think Trump would comply with any subpoena or court order he didn't want to face, because it would reveal criminality, or at least something too embarrassing?
Of course he wouldn't. Kavanaugh increases the likelihood that refusal to comply with a subpoena, or oversight more generally, would be given the sheen of court legitimacy, but Trump would never comply with a court order that forced disclosure of a secret he's trying to keep for the sake of hiding his criminality.
You don't seriously think you're ever actually going to see the Mueller report, do you?
Suppose the House flips in 2018. Democrats will attempt to reassert Congress's oversight authority. Trump will not comply with congressional oversight. There will be a court battle. If Trump wins, legislative oversight, as a concept in American politics, dies. If Trump loses, he still refuses to comply, if the demand is for something like his tax returns.
Yes, I realize what I'm typing here. We have had presidents defy the Supreme Court before. What would happen? I don't know. "John Marshall has made his decision. Now let him enforce it." Trump loves to puff up his chest like a phony tough guy, and he plays the racist-populist demagogue role, even though he was a draft-dodger rather than an actual General. There's history here, though.
So, what is at stake, with regard to congressional oversight? Only this: whether or not we tell ourselves, falsely, that it is still a thing. It is in that illusory state in which Democrats had the right, in principle, to filibuster George W. Bush's judicial nominees in some unspecified circumstances, post-Gang of 14 deal, as long as they never actually used that right, knowing full well that Republicans would filibuster Democratic nominees as soon as the shoe was on the other foot. A right you can never exercise, and only exists in principle, isn't a real right. That's congressional oversight right now. It's a thing that exists as some theoretical construct, that Democrats could use in an alternative timeline in which Republicans hadn't already fatally wounded the rule of law, but that will never actually successfully happen in this timeline with a Republican president. Either the GOP keeps the House, and no oversight happens, or the House flips, Democrats try to use those oversight powers, and we observe that they can no longer be used because the Republican Party no longer recognizes the constitutionality of the practice, when the president as an "R" after his name.
I don't know what the sequence of events is in that latter eventuality. I object to the concept of believing in comforting illusions, though. I would like checks-and-balances to exist, but they don't. At some point, we do need to confront that reality. It's gonna get ugly. What's at stake in 2018? Whether or not we start dealing with that.
And I'm not done.