Veteran GOP operative Jim Ellis points to the second poll so far this cycle showing Flake in big trouble — not of losing to a Democrat, but of losing his primary. Neither poll is particularly impressive in its pedigree, but the idea that Flake is in trouble is by no means far-fetched.

The first poll, by Remington Research in November, had shown Flake with a 19-point net-negative approval rating among likely Republican voters, and that he trailed Republican state Treasurer Jeff DeWitt, 42 to 33 percent in a head-to-head.

The new poll was apparently conducted on behalf of former state Sen. Kelli Ward, who unsuccessfully challenged John McCain last year and is already in the race to challenge Flake. The numbers don't come from a particularly reputable firm, and the result — Ward 30, Flake 23 percent — is a bit shady if only because there are so many undecideds.

On the other hand, as recently as September, Morning Consult's ongoing series of surveys on senators' approval ratings had Flake at a reasonable 41 percent approve/35 percent disapprove. But that was five months ago. Flake, a former House member who just barely won his first statewide election in 2012, has more than 17 months to recover, as Arizona's primary is in late August. But he's probably also the weakest Republican incumbent facing re-election in 2018.

Flake first irked conservatives by moving toward the center since arriving in the Senate. But that's nothing compared to the way he irked Trump supporters with his outspoken criticism of the GOP nominee. Hillary Clinton even used some of his comments from a television interview in an ad, in which Flake is quoted saying he "can't vote for Donald Trump given the things he's said." Trump attacked Flake in turn, stating he would lose his re-election, and he's never been in a better position to make that happen.

As of January 1, Flake's campaign bank account only contained about $600,000. That could be just because it would have been bad form to raise money while his senior colleague, John McCain, was running his re-election race. But he's going to need quite a bit of cash for the battle that's coming up.