Read those two posts for extensive historical context, drawing parallels between these midterms and those of other disjunctive, end-of-an-era administrations -- Carter, Hoover, Pierce (and Buchanan), John Quincy Adams, and John Adams.
Those posts cover the "why" as well as the "what" -- the dominant party of a historical period, such as the GOP during the Reagan era, begins and ends its disjunctive phase with full control of government, including limping across the finish line during the midterm election. Voters do not transfer control during the midterm, as is usual, because the big issues are at stake during a disjunctive phase, not the minor issues that draw attention during the regime's heyday. And the opposition does not, or cannot, campaign on these revolutionary issues during an off-year election.
This year's opposition Democrats are not offering enough of a radical change to counter the escalation of militarism, the record widening trade deficits and de-industrialization, soaring numbers of cheap labor immigrants, falling real wages and deteriorating standard of living, and last-ditch inflation of the bubble economy by cutting taxes without paying for them, leading to yet another record year for our national debt. So while voters will not be pleased with the GOP's performance so far, they will not transfer power to the Democrats...
The big story is the rise of the Bernie candidates, just as 1854 saw the first-ever explosion of the realigning Republicans. They began with no one in the Senate before 1854, and picked up 3 (out of 62). And they began with only 4 in the House and ended with 37 (out of 118). Whether they're affiliated with Our Revolution, Justice Democrats, the Democratic Socialists of America, or are their own economic populist and anti-imperialist, these candidates are the clear wave of the future...
For now I'm keeping the chances of this "two disjunctive terms" scenario below 50%, but when the Democrats fail to pick up either house of Congress in the midterms, I will raise it above 50% if the psychotic centrists double down on ignoring the major issues and offering only the status quo, at a time when it is rapidly disintegrating. When the status quo was strong, during the '90s, it was feasible to offer their take on the status quo and win. But by now, Reaganism is dead, and they must offer a wholly different system -- at least as radically different as the system that Trump campaigned on in 2016.
Democrats will keep losing until they internalize what Trump ran and won on in 2016 -- a repudiation of Reaganism (neoliberalism) by the GOP's own hardcore primary voters, crossover voters, and the leading candidate himself. Wanting to close the borders and deport illegals is only one part of the broader reversal of neoliberalism, which relies on open borders for cheap labor, and return toward Progressive Era or New Deal outcomes, which both relied on closed borders to protect domestic labor against boatloads of foreign scabs (immigration peaked during the 1910s).
Almost no Republicans support restricting immigration, let alone closing the borders or deporting illegals or building a wall. With full control over government since Reagan's initial victory, they have flung the gates wide open, given amnesty to millions, and lost sleep trying to import millions more. They are about maximizing profits for labor-intensive sectors of the economy, not airy-fairy bullshit like "maintaining white supremacy". That means they need endless cheaper and cheaper labor, and that's not going to come from here, so it must come from somewhere over there. It's that simple.
The very small handful of Republicans who do support restricting immigration, however, do not come to that policy from a populist standpoint -- Steve King of Iowa does not want single-payer healthcare, soak-the-rich wealth tax to pay off the debt, de-globalizing our military, and so on and so forth, like Trump has promoted before becoming president (and getting promptly cock-blocked by the GOP Establishment). King wants the same ol' Reaganite garbage of tax cuts, deregulation, and imperialism, but he doesn't want it to go so far that "maximum profits" leads to 100 million new immigrants. Too bad, numbnuts -- that's exactly where laissez-faire and profits uber alles leads us.
To win dominant status, rather than an occasional opposition victory whose effects are fleeting, the Democrats must campaign on populism rather than elitism, and anti-globalization -- their spin on Trump's nationalism. If they refuse to adapt to the new climate of the Trump era, they will lose in 2020. No matter how minuscule Trump's accomplishments will be by then, all he has to do is say, "Hey folks, at least I'm promising the right policies of anti-globalization -- the Democrats won't even give you their word. They're promising to open the borders, send more factories overseas, and stay bogged down in the Middle East, NATO, and everywhere else."
At that point, he may even get away with the whopper he's been testing out about how it's the Democrats rather than the GOP who wants to gut Medicare and Social Security. If the Democrats refuse to campaign on single-payer, if they refuse to campaign on collective bargaining against the pharma cartels to crush the prices of drugs way down, if all they do campaign on is "protecting pre-existing conditions," then maybe voters will conclude the Dems don't actually give a shit about Medicare after all.
As of midterm Election Day, I see zero sign that the Democrats are going to try to steal Trump's own popular winning issues from him, leaving him with powerful weapons to beat the shit out of them with in just two years. It doesn't matter if he himself runs, or if he campaigns on behalf of Pence, Haley, or whoever.
I think it will take a second disjunctive, end-of-an-era term to finally break the Democrats' stubbornness and force them to do what the voters want, and shut their mouths about the fake idiotic crap that alienates 95% of the country. Otherwise, they are effectively extinct as a party.