With every new twist and every new scandal, I keep saying that I don't think Trump will be impeached, or lose support from Republican officials. The response I get, from several quarters, is that if public support drops below a certain level, Republican officials will turn on him. My point, though, is that Republican officials are preventing public support from dropping to that level by refusing to condemn Trump. The process is a dynamic one, or, in the jargon favored by economists, an "endogenous" one, meaning generated internally. "Endo," from inside, and, "genous."
Anyway, suppose that Trump's public approval numbers dropped to 10%. Would Paul Ryan and Mitch McConnell keep doing the pseudo-condemnation two-step where they pretend to be bothered by something, and then the next day, talk about how awesome Trump is? Probably not. They might even agree to impeach him for, ya' know, obstruction of justice, for starters.
My point, though, is that if Trump's approval has gotten down to 10%, it means that Ryan, McConnell and a lot of other Republican leaders have already stopped supporting Trump.
The basic reference here is a critical book for understanding public opinion: The Nature and Origins of Mass Opinion, by John Zaller. Zaller's model is one in which people are essentially taking cues from elites. The opinion you give in response to a survey question is the balance of liberal and conservative considerations in your head at the time. What determines that? You receive cues. Those cues are sent by elites, and you selectively accept or reject them, based on how consistent or inconsistent they are with what you already hold. So, dislodging existing opinions is kind of hard, but possible when all of the cues move in one direction.
So, you get big, unidirectional movement in public opinion when all of the signals are going in the same direction. If even Paul Ryan, Mitch McConnell, Fox News, etc. said that Trump is a crook and a Russian stooge, then the Republican electoral base might accept it, and his approval numbers would drop even more. Right now, if you look at Gallup or other tracking data, you will see that Trump's support seems to be hovering just shy of 40% in the polls. As long as those 40% can still get signals of positive support for Trump, though, their positive opinions will be reinforced. This only changes if Republican elites decide to stop supporting Trump. That's why their strategy can work.
Could an economic crash tank Trump's numbers? Probably. Trump would lie, and say that the economy isn't tanking. He could stare down the barrel of the 2008 collapse and claim that it is the greatest economic boom in the history of the world. "Fake news!" he'd whine... Fox might even run with it, but it would be an interesting test of just how far lying can get you these days. However, scandals only matter if elites tell the voters that they matter. The Republican Party has decided that Trump's scandals will never matter. Therefore, to a lot of voters, they will never matter.
Raw speculation time: what would it take to get Republican officials to turn on Trump? Trump turning on them. Trump is a vindictive sack of shit, and he is getting more and more pissed that Congress isn't making any policy progress. If he pulls something because of their failure to do anything on healthcare, or over failure to fund his wall... If Trump makes this personal, maybe then the legislative caucus turns on him. I figure if I give you a bunch of Zaller today, I'm justified in some bullshit speculation. The intellectual merits balance out, and I'm still in the black.