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Sen. Jeff Flake (Ariz.) is not the first prominent Republican elected official to be openly critical of President Trump, but he may be the first one to have become significantly more critical since Trump was elected, not merely objecting to a specific decision or statement or tweet, but raising serious doubts about this entire presidency. That makes what he’s doing right now important — but the limitations of his criticism (and that of other Republicans) is a warning to us that there’s only so far we can expect members of the president’s party to go, no matter how bad things get.

Today Flake releases his new book, “Conscience of a Conservative: A Rejection of Destructive Politics and a Return to Principle.” In an excerpt published by Politico, he seems to reject the entire premise of Republican support for Trump:

If by 2017 the conservative bargain was to go along for the very bumpy ride because with congressional hegemony and the White House we had the numbers to achieve some long-held policy goals—even as we put at risk our institutions and our values—then it was a very real question whether any such policy victories wouldn’t be Pyrrhic ones. If this was our Faustian bargain, then it was not worth it. If ultimately our principles were so malleable as to no longer be principles, then what was the point of political victories in the first place?

But lest liberals get too excited, Flake’s recommendations for what Republicans should do in response to this realization that they’ve gone down the wrong path are pretty weak:

First, we shouldn’t hesitate to speak out if the president “plays to the base” in ways that damage the Republican Party’s ability to grow and speak to a larger audience. Second, Republicans need to take the long view when it comes to issues like free trade: Populist and protectionist policies might play well in the short term, but they handicap the country in the long term. Third, Republicans need to stand up for institutions and prerogatives, like the Senate filibuster, that have served us well for more than two centuries.

So: occasionally speaking out against demagoguery, sticking to conservative policy principles and supporting the filibuster. Not exactly a program of full-throated opposition.

Should we have expected any more? Let’s not forget that Flake is a conservative Republican — the American Conservative Union gives him a lifetime rating of 93 percent — so it isn’t as though he’s going to start co-sponsoring Democratic bills. Flake also isn’t taking all that much of a political risk by speaking out against Trump. He’s one of the few vulnerable Republicans up for reelection next year, in a state that Trump won by only 3.5 points. His approval ratings are extremely poor, so becoming a high-profile Republican critic is a good way of showing voters that he’s an independent voice. It’s also personal: Trump was reportedly livid at Flake’s refusal to endorse him in the presidential campaign, and the White House has been encouraging potential primary challengers to him.

But there’s no contradiction in saying that Flake’s criticism of Trump isn’t some kind of awe-inspiring profile in courage yet is still significant. He differs from most of his colleagues in that he’s not just shaking his head and saying he’s “troubled” about the latest Trump outburst. Instead, Flake is offering a critique of the entire Trump presidency and the bargain he and his fellow Republicans made with their own consciences when they decided to stand behind him, or at least acquiesce to him.

If some coastal elitist liberal like me makes an argument like that, conservatives will dismiss it out of hand. But it’s harder to ignore when it comes from a conservative in good standing like Flake (who I’m sure will nevertheless be condemned as a RINO traitor on talk radio). Even if he might not win too many converts right away, he could at least plant seeds of doubt that could grow over time.

Cracks in partisan unity give people — both officeholders and ordinary citizens — permission to open themselves to criticisms they might not have been willing to entertain, and even eventually to action, if the time ever comes when it’s necessary. We’re a significant way from there, but it isn’t hard to imagine a moment where even more awful (or unlawful) behavior on President Trump’s part is revealed, and Republicans hold his future in their hands.

At that moment, the question we often mull over — can Trump hold on to his party? — will become absolutely vital to the future of the country. There will always be forces pushing toward party unity, like the impulse to rally around the president of your party when he’s under attack from the other side, or the fear of the political fallout from something like impeachment. But if Republicans haven’t even considered stepping away from Trump, they’re far less likely to do it at the instant of his greatest peril.

In the meantime, some Republicans are already separating themselves from him, albeit in limited ways. They’re rejecting his threat to destroy the individual health insurance market out of spite. Many expressed displeasure with how he’s trying to torture his attorney general, Jeff Sessions. They passed a bill imposing sanctions on Russia over his objections. Some of them criticized his decision to ban transgender Americans from serving in the military.

Those are all specific and limited cases, and we should be cautious about making too much of them. The GOP is still emphatically behind Trump, to its everlasting shame. But we can no longer take for granted that every Republican will support Trump in whatever he does. If I were the president, it would be making me a little uncomfortable.