On Thursday, Mr. Cruz told me his debt procedure was a matter of principle, though he acknowledged an “additional benefit” was the “transparency” he’d forced on Republicans. He told me he had not “spoken to anyone at SCF in months.” However, when I asked if anyone on his staff had been in contact with outside groups about his debt-ceiling procedure, he acknowledged: “My staff periodically speaks with people across the conservative movement.” He added, “But the debt ceiling vote occurred suddenly and it was a surprise to everybody when Republican leadership asked every Republican senator to consent to letting Harry Reid raise the debt ceiling.”

In addition to Mr. McConnell, conservative groups are targeting senators John Cornyn (Texas), Pat Roberts (Kan.), Thad Cochran (Miss.), and Lindsey Graham (S.C.). While the primary challengers aren’t likely to win (Mr. Bevin is trailing by 25 points), the attacks are hurting incumbents’ general-election prospects.

None of this is about substance. If political principle were at stake, one would assume these outside groups—so keen on purity—would have already dropped Mr. Bevin. It came out recently that he had once praised the very bank bailouts that he has been slapping Mr. McConnell for supporting.