As a rule, the Republican-led Senate effectively serves as a rubber stamp for Donald Trump’s nominees. The president is told whom to pick for various offices; the White House sends the choices to Capitol Hill; and GOP senators vote “aye.” It’s an assembly line that Democrats lack the power to interrupt.

But once in a great while, Republicans balk, and when that happens, it’s worth pausing to understand why. The Wall Street Journal reported overnight:

President Trump on Tuesday withdrew his nomination of a former top police union official to head the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives after some Republican senators expressed concern that he would restrict the rights of gun owners. Chuck Canterbury, the former head of the national Fraternal Order of Police, wouldn’t have had enough Republican support to advance beyond the Senate Judiciary Committee, people familiar with the matter said.

The White House officially pulled Canterbury’s nomination yesterday afternoon — there was no explanation given — and it’s not clear when, or even whether, a new ATF nominee will be chosen.

At this point, you might be thinking that Canterbury must’ve been ridiculously awful in order to draw objections from Senate Republicans who ordinarily do as Trump directs them to do, but in this case, it’s not that simple. The WSJ report added:

He tangled in his confirmation hearing with Republican senators who were frustrated that he wouldn’t clearly define his views on gun-control measures such as expanding background checks for prospective buyers and a ban on assault rifles. He was head of the FOP when it took positions supporting more-rigorous gun-control measures including an expansion of background checks.

And that’s why this makes sense. Trump chose an ATF nominee whose views on gun policy conflicted with Republican orthodoxy — and while GOP senators are nearly always inclined to go along with the White House’s wishes, this was a step they were unprepared to take.

It’s a familiar problem. In 2013, the Senate just barely confirmed federal prosecutor B. Todd Jones to serve as the ATF director, which, in a way, was a historic breakthrough. Since the position became a Senate-confirmed job in 2006, Republicans wouldn’t allow anyone to hold the office. Jones was the first — and only — person to ever win confirmation.

So long as there’s a GOP majority in the upper chamber, that seems unlikely to change.