Sen. Jeff Flake (R-AZ) issued a dire warning to the Republican Party on Tuesday: If you don't do everything in your power to stop President Donald Trump now, future generations will never forgive you.

Someone should tell that to Jeff Flake.

Flake made a show on Tuesday of announcing his retirement in 2019 in order to speak his mind freely until then. But there was nothing stopping him from doing both. He could honestly express his horror with Trump’s leadership and agenda without giving up the power he currently has to alter that agenda and chart a different course for the GOP.

It’s becoming a pattern. At this point, the Senate’s two most substantive Republican Trump critics (Flake and Tennessee’s Bob Corker) have already announced their retirements; a third, Sen. John McCain, has brain cancer. Flake is stepping down explicitly because he believes the Republican base loves Trump, and Trumpist populism, more than it loves the brand of conservatism Flake holds dear.

If Flake is worried about what President Trump will do to America unless stopped, he can’t stop him by ragequitting the Senate. And if Flake really cares about the people he worries Trump will hurt, he has an obligation to stay and fight.

Flake and Corker are both worried about specific things Trump might do — which they, once retired, can’t stop

Most elected Republicans have expressed discomfort, or even disgust, with Trump’s style. They would rather Trump spent a little less time attacking Gold Star families, for example. But they limit their criticism to what Trump is saying on Twitter, on television, and in interviews — not the decisions he’s making as the most powerful man on earth.

That isn’t true, however, of Trump’s critics in the Senate. Both Flake and Corker have voiced specific and vehement concerns about what Trump wants to do with the US government and how he wants to go about it. But they are voicing those concerns at the moment they step away from positions that would allow them to stop their fears from becoming reality.

Flake has been the Republican who shows the least respect for any part of Trump’s immigration agenda. He’s supportive of a bill to legalize immigrants currently protected by the DACA program Trump is winding down, with only a few of the enforcement concessions the Trump administration asked Congress for last month. He’s been dismissive of a border wall.

Arguably more importantly, though, while other Republicans have endorsed the idea of an overhaul to the legal immigration system to reward "merit-based" immigration —even if they quibble with the specifics of Trump’s favored bill in Congress — Flake has questioned it. He signed a New York Times op-ed in August telling the story of a family ranch hand who, despite being "low-skilled,” was just as deserving of American citizenship as Flake himself.

Donald Trump can’t overhaul legal immigration without Congress. Jeff Flake is leaving Congress. And private citizens don’t get a say in the legislative process — especially when they’ve literally said their opinions are too toxic for anyone who covets reelection to hold.

Corker is in a similar position. He doesn’t appear to have specific policy disagreements with Trump, but he’s made it clear that he thinks the president is too unreasonable and irresponsible to run US foreign policy—and that in a worst-case scenario, Trump might get us into World War III.

Perhaps no senator, even the chair of the Senate Foreign Relations Committee, could stop the president from dragging the US into a disastrous war. But all things being equal, he’d certainly have more opportunities to do so than an ex-senator.

At worst, Flake would have six more months of his colleagues listening to him

But it's not just that Flake is running away from the fight himself. It's that he's telling his colleagues they shouldn't fight Trump either — at least if they value their political futures.

"The path I would have to take to the Republican nomination," Flake said — the race to represent his own party in his own reelection campaign — "is a path I cannot in good conscience take."

Would he really have to take such a path? We don't actually know if a Republican incumbent can get unseated for being insufficiently loyal to the president. We have polling telling us it's likely, but in the wake of an election that proved that state polls weren't always reliable and unlikely outcomes were hardly unthinkable, that seems a proposition worth testing.

But sure, Flake might lose. So might Corker. They would end up, in other words, with exactly as much say in government as they are going to have anyway, as ex-senators who are ex-senators because they didn't think they retained their party's support.

In the meantime, though, they’d be senators in good standing, just as worth listening to as anyone (or even more influential, since Republicans would likely want to protect their colleagues in the hopes of keeping their majority). Those months matter. The Senate needs to figure out whether it’s going to reimpose Iran sanctions. On March 5, if Flake and company haven’t passed a bill yet, thousands of DACA recipients will start becoming vulnerable to deportation every day. And there’s at least one government shutdown deadline coming up — an opportunity for any Republican senator who wants to force the party to bend to his wishes.

As lame ducks, on the other hand, Corker and Flake now have no reason for any other Republican to listen to them. Flake has all but said that you can’t have a career in Republican politics while standing up to Trump.

Maybe some other senators, who weren’t facing reelection in 2018, might join Flake on the floor to criticize Trump or stand up for Corker — and Senate norms — when the president mocked him on Twitter. Maybe some other members would decide to run as dissidents as 2018, as part of a cohort rather than the sole target of a Bannonian deluge — and maybe a few would win.

Jeff Flake is telling America that Trump needs to be stopped. But Flake will never know what would have happened if he had really done all he could to stop him.