But a deeper dive into the poll's findings show his feud with Trump has clearly alienated Republicans.

I asked the pollster to pull numbers on just Republicans, and it turns out Corker's approval is now actually lower inside the GOP than in the state as a whole. Just 37 percent of Republicans say they approve of Corker, while 49 percent disapprove. That's much worse than in February, when Corker's approval was 61 percent among Republicans, versus just 21 percent who disapproved.

In other words, he was plus-40 before; now he's minus-12.

And the Republicans who still approve of Corker may remain in his corner because they simply haven't tuned in. Among Republicans who say they have paid “some” or “a lot” of attention to the Corker-Trump feud, two-thirds (67 percent) now disapprove of Corker. Among those who have paid less attention, just 38 percent disapprove.

Corker, who had worked with Trump extensively as chairman of the Senate Foreign Relations Committee, announced his retirement a month ago and has since steadily upped the ante on criticizing Trump. He has now suggested Trump has put the United States on a path to World War III and said he is “debasing” the country. I wrote this week that Corker sounds a lot like he's making the case for removing Trump from office.

Tennessee Republicans don't appear to appreciate that.

But Corker is hardly the first Republican to see this happen. Before him, Sens. Ted Cruz (R-Tex.), John McCain (R-Ariz.), Jeff Flake (R-Ariz.) and Mitch McConnell (R-Ky.) all saw their numbers decline — often precipitously — among GOP voters after they ran afoul of Trump. Flake announced this week that he was retiring because he couldn't win a primary in Trump's Republican Party. Sen. Dean Heller (R-Nev.), who balked at supporting a health-care bill Trump wanted passed, also appears to be in jeopardy in his 2018 primary.

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Challenging Trump can get Democrats on board with you, though, as McCain has shown us. And indeed, Corker has picked up support from non-Republicans, keeping his overall approval rating in reasonably good shape.

But for fellow Republican lawmakers who are generally more worried about stoking primary challenges by challenging Trump, Corker's decline among GOP voters will surely serve as a warning sign.