The Republican senators went to the White House and saw a president so repetitive and rambling, some thought he might be suffering from early Alzheimer’s. But they know which way the wind is blowing. They gave him a standing ovation.

This is an argument that will make liberals' heads spin. How could Trump have “won” anything when a Republican senator, Jeff Flake of Arizona, delivered such a thorough rebuke of him and his worldview on the Senate floor?

It's one thing to make a moral judgment of Trump, though. It's another to make a practical, political one about the state of the GOP. And what followed Flake's, Sen. Bob Corker's (R-Tenn.), Sen. John McCain's (R-Ariz.) and former president George W. Bush's stark criticisms of Trump over the last week or so is perhaps most telling of all: Basically nothing.

As Brooks notes, the GOP chose unity — or, at least, the illusion of it. Corker's and Flake's GOP colleagues largely poo-pooed their broadsides against Trump as petty personal politics and Twitter drama, glossing over their warnings of World War III and the undermining of our entire political system. They basically decided this was not the time for a revolt — not with tax cuts just over the horizon.

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Their reasons are clear: The party is still largely behind Trump, and pretty much everyone who runs afoul of him sees their stock among Republicans plummet. That's not a recipe for reelection, now, is it? And they can still enact a conservative agenda! (At least theoretically.)

Given all of that, we can say three things about the 2020 GOP presidential primary:

Trump is still in relatively little danger, BUT . . . His grip on the party is weakening somewhat, and . . . The likelihood that someone will run against him is rising.

Why do I say No. 2? Because Trump's approval rating among Republicans, while still around three-quarters of them, is falling. A Pew survey this week also showed most of the base doesn't truly align with Trump on policy or on his conduct in office. They largely approve of him and have stood by him, but they don't truly love him. In fact, they like Vice President Pence better, and about twice as many base Republicans strongly dislike Hillary Clinton as strongly like Trump. The unifying factor is what Trump stands against, not him personally.

There is also that poll released by Trump's own pollster, Tony Fabrizio, which showed the president at 50 percent in a hypothetical primary, with Sen. Ted Cruz (R-Tex.) at 14 percent and Ohio Gov. John Kasich (R) at 10 percent. That's a big lead if the field is crowded, but 50 percent is not strong territory for an incumbent. It suggests the door is open in a one-on-one race.

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With all of that out of the way, here's a brief rundown of who could challenge Trump in a primary, with No. 1 being the most likely to succeed — albeit still quite unlikely at this point.

5. Sen. Jeff Flake (R-Ariz.): Flake made big news this week for his stemwinder against Trumpism, but he's been a consistent critic, not backing Trump in 2016 and authoring a book highly critical of him. Flake also left open the possibility of running in 2020 this week, saying “that's a long time away.” But judging by his media appearances this week, I'm not sure Flake is the strongest messenger in a race that may demand a big personality with a quick wit to counter Trump.

4. Mitt Romney: The other four names on this list were pretty easy to assemble. This one is more outside-the-box. But hear me out: Romney was a huge Trump critic dating back to the 2016 campaign, delivering a major speech against him. He also considered running again in 2016, before stepping aside for Jeb Bush. And he's still clearly got the bug, given he's considering a 2018 Senate campaign in Utah if Sen. Orrin G. Hatch (R) retires. Romney would basically walk into the Senate, and would immediately have a platform for speaking out against Trump.

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3. Sen. Ted Cruz (R-Tex.): Cruz has been conspicuously quiet ever since the 2016 Republican National Convention, when he declined to speak positively about Trump during his speech and later said he would not be Trump's “servile puppy dog.” His numbers in Texas sank afterward, and he's got a 2018 reelection campaign to worry about, which is perhaps why he's been so quiet. In some respects, Cruz appears to have reconciled with the Trump team: Cruz and his family dined with Trump at the White House in March, and former White House chief strategist Stephen K. Bannon reportedly says he's the lone Republican incumbent in 2018 against whom he won't back a primary challenge. But I wouldn't be so sure he's not still available for 2020, under the right circumstances. And if he ran, he'd be formidable.

2. Ohio Gov. John Kasich: Like Flake, Kasich has left the door cracked to a run in 2020. And unlike a lot of these Trump critics, he hasn't seen it damage his personal image, which remains strong on Ohio. Kasich didn't win anywhere in the 2016 GOP primary, though, besides his home state. So the environment -- or the candidate himself — would need to be quite different.