Conservatives are tremendously vexed now by Sen. Tim Scott of South Carolina.

A good politician knows not to stab his colleagues in the back. And, if on a rare occasion, he does find a truly compelling reason to change his mind after assuring colleagues and the public otherwise, he at least owes a full and cogent explanation for the change.

Scott has now reversed course twice, both times at the proverbial last minute, on whether or not to confirm judges nominated by President Trump. In both cases, critics wrongly said the nominees had troubling histories concerning racial matters. Scott, the Senate’s lone black Republican, seemed satisfied both times that the allegations were meritless, but then he switched gears and torpedoed the nominations anyway.

Yesterday he did so with regard to federal district court nominee Thomas Farr of North Carolina. Shortly before Scott announced his change of heart, I explained here yesterday that a 1991 memo effectively exonerated Farr of material involvement with an ill-advised postcard mailed to black voters by the 1990 re-election campaign of then-Sen. Jesse Helms. Indeed, the memo, backed by the testimony of others, showed that Farr advised against any postcard mailer related to “ballot security” that year.

The odd thing is that Scott said as much in an interview with Fox News’ Shannon Bream.

“If [Farr] was the architect of that nasty, racist campaign, I would have been a ‘no’ without any question,” he told Bream Wednesday, when he was still in the “yes” camp. “What I found so far from appointees of the Obama administration to my conversations with the author of the memo is that [Farr] was, in fact, not the architect of the campaign and that the character witnesses from the Obama administration coming forward on behalf of Tom Farr have been pretty strong.”

Within 24 hours, he had changed his mind. The only explanation he offered was that the same memo he had already cited in Farr’s favor now “shed new light on Mr. Farr's activities. This, in turn, created more concerns. Weighing these important factors, this afternoon I concluded that I could not support Mr. Farr's nomination.”

Scott did not identify what those new “concerns” were. He offered no explanation why a memo exonerating Farr should now be seen as disqualifying him.

The only other information regarding Farr in that memo was that he had been the coordinator of an earlier, different postcard mailer, in Helms’ campaign six years earlier, in 1984.

But unlike the 1990 mailer, which had wording seen as dissuading black voters from participating in the election, the 1984 postcard to black voters featured an endorsement of Helms by black leaders — urging, not discouraging, electoral participation. If the mailers were returned as undeliverable, the campaign could use them to ensure that no votes had been cast by non-residents — but that was not just legal, but standard practice at the time and still an officially encouraged part of voter laws in many states. Unlike in the 1990 effort that Farr had nothing to do with, the 1984 one was not challenged by the Justice Department.

In fact, just this year, in Husted v. A. Philip Randolph Institute, the Supreme Court ruled that undeliverable mail legally can be part of a process of determining if a voter no longer resides where he is officially registered.

Scott ought to change his mind on Farr and vote to confirm him this month. He has offered not a single, specific shred of logic or principle for his opposition.