So imagine Kavanaugh (or, in the unlikely event his nom fail and another Trump nom gets confirmed) gets on the court and now there's a conservative firewall for a generation against any law or policy coming out of unified Democratic political branches...
Say that 5-4 conservative majority starts striking down all sorts of broadly-mandated liberal legislation. We've already heard lots about court packing, court shrinking, adjustable court membership, etc.
But I'm looking past FDR's First New Deal fight and 1937's Court Packing plan and going right to the source of where we are at this very moment:
It took until Jackson for Marshall's zombified Federalist Party veto to be openly and easily defied. Take it away, Justice Breyer: 
That case was in 1831. It took the Jefferson/Jackson revolutions 30 years from the time Marshall arrived on the bench - from the time the Federalists were rendered a permanent political minority - for a President to openly defy the Supreme Court
Marshall died four years later and Jackson got his man in Chief Justice Taney to turn the Court's tide away from a dead politics of the Founding towards the destructive politics of the antebellum years.
Back to Justice Breyer:
If a conservative court strikes down affirmative action, limits states' repro rights protections, etc, while WH/Cong are held by Dems, I can see the feds refusing to enforce SCOTUS decrees, contra Ike's calling in nat'l guard to fight massive resistance to desegregation
Similarly, I can see a Dem WH/Cong simply ignoring SCOTUS decrees striking down federal legislation (Medicare for All, Federal Jobs Guarantee, etc etc).
Now of course politics is fluid and just as a permanent conservative SCOTUS majority could sweep in a long-lasting Democratic political majority, so could that political majority's resistance to what they see as an illegitimate law-ruler backfire at the polls.
But say politics remain stable: demographic realities, stronger Dem turnout, etc, give the liberal side of the ledger an insurmountable political leverage, while SCOTUS maintains a 5-4 conservative majority (that assumes Thomas gets replaced by a conservative, too)...
Then this is where, contra my below tweet from when Kennedy retired, Chief Justice Roberts does, indeed, become the swing vote.
Roberts, as we all love to note, is an institutionalist. Now it's a very, very high bar for him to invoke such institutionalism, because it's only really happened with the Obamacare case, which had very loud echoes of 1937
Roberts would certainly turn to his institutionalism in cases where his fifth vote for what he believed in would compel the political branches to act like Andrew Jackson rather than Ike. He wouldn't permit that doomsday scenario.
So, in sum: if things ever come to such a head where the political branches, backed by the people, would be willing to ignore or defy a SCOTUS decision, Chief Justice Roberts would not make such a decision. Unless he's made irrelevant by a 6-justice or more conservative majority.
If Trump wins in 2021, a 5-4 conservative court will likely become a 6-3 conservative court, whether Thomas goes voluntarily or RBG can't hold on till she's 91 in January 2025. But then again:
OK, I'm done. Thank you for reading.
Wait I'm not done.
All of the above was predicated on a reality that pre-existed Kavanaugh's second hearing, in which Kavanaugh is a fungible judge from Trump's List.
Kavanaugh's performance at his second hearing and the process surrounding his potential confirmation have now further poisoned the very foundation of goodwill the rule of law requires, as Breyer identified in this 2003 speech, three years after Bush v. Gore
Now I happen to celebrate the kind of open partisanship Kavanaugh displayed not only because it ripped the mask off of what these proceedings really are, but also because I think "judicial temperament," at least at this moment, is a canard
But I also recognize that I'm in the vast minority of those who see the rule of law and power politics, operating together across and within our branches of government, as consistent with our very constitution, for better or worse.
And confirming Kavanaugh in the way he is looking to be confirmed, after he conducted himself the way he did, supercharges the possibility for a terrible breakdown of the court's countermajoritarian legitimacy
