Sen. Richard J. Durbin (D-Ill.) and President Trump speak during a meeting with lawmakers on immigration policy in the Cabinet Room at the White House on Tuesday. (Jabin Botsford/The Washington Post)
Opinion writer

It is not just that President Trump refuses to read. The problem that Republicans must come to grips with is his inability to understand, process and respond in a rational way even when information is presented verbally. That was on view to an alarming degree at Trump’s open-to-the-press meeting Tuesday with lawmakers on immigration.

The Post reports:

For a moment, Democrats thought they had struck an unexpected deal with President Trump. Trump had previously insisted that any deal protecting “dreamers” — undocumented immigrants brought to the United States as children — should also include border security and/or a border wall. But he now says that he would support a “clean” bill protecting dreamers, and then take up comprehensive immigration reform later.

“What about a clean DACA bill now, with a commitment that we go into a comprehensive immigration reform procedure?” asked Sen. Dianne Feinstein (D-Calif.). Trump responded: “Yeah, I would like to do that. I think a lot of people would like to see that.”

The problem? Trump didn’t know what “clean DACA bill” meant. House Majority Leader Kevin McCarthy (R-Calif.) quickly interjected and made clear that Trump believes a “clean” bill would include border security. Except that’s not at all what a clean bill is; that’s a compromise bill. A clean bill, by definition, only has one component to it.

In other words, asked whether he wants chocolate or vanilla ice cream, he responds, in all seriousness: yes. But chocolate means it’s not vanilla, sir. Too bad, that’s a detail others will work out. We’re going to have a magnificent dessert — the best!

“If anything, the whole mess showed pretty vividly just how utterly disengaged Trump is in the finer details of policy discussions,” The Post reports. “Which is exactly the perception that he has recently fought against.” The lack of comprehension may be unintentional (i.e., he’s trying but can’t understand relatively simply policy ideas), or it may be intentional (i.e., he doesn’t need to understand policy because he couldn’t care less). Either way, his lack of comprehension may not make him insane, but it does make him so unfit to be president that one can understand the uniform dismay that staffers expressed to fly-on-the-wall author Michael Wolff. (The Post continues: “Trump has repeatedly assured us that he knows this stuff better than almost anyone and that he’s the world’s preeminent negotiator. What we saw Tuesday was neither of those things.”)

It is wrong to call this merely a lack of interest in details. That would be leaving the allocation of border spending to his advisers. No, this is a fundamental inability to understand who is on which side and why, what the implications of various policy choices might be and what substantive compromises are possible. In no real sense is he directing or expressing policy outcomes.

And this is in the context of domestic policy. Imagine how frightful it must be when talking through foreign policy decisions. Even worse, imagine how he comes across to both allies and foes when he meets them one on one. They surely must figure out that the American president is out there to soak up compliments and applause but not to defend U.S. positions or interests.

What matters is not that he seems to have moments of clarity but rather that so much flies over his head. No wonder the last person he talks to determines the outcome; he may not remember anything that came before or have any understanding of the material so he can make informed decisions. This is not masterful management or the artistry of the deal. It’s a frightful portrait of someone impersonating what a real president might sound and look like. Trump gives the pretense of comprehension, but there is little if any indication that he is capable of making rational decisions based on available information. That doesn’t mean he’s mentally ill; it just means the voters made an awful, awful mistake.

Read more by Jennifer Rubin:

Morning Bits: ‘Write about it fast because it won’t last long’

What didn’t Republicans want you to see in the Fusion GPS transcript?

What Lindsey Graham and the rest of the GOP toadies miss

Why the White House will never silence the buzz about Trump’s mental fitness

Will Democrats stop Trump’s cruel use of immigrants as pawns?

Trump on the stand: The greatest political and legal peril he’s ever faced