Sen. Bob Corker (R-Tenn.). (Melina Mara/The Washington Post)

IT IS HARD to say what is most damning about the process Republicans used to produce their $1.5 trillion overhaul of the tax code. Is it the suggestion by critics that a key senator dropped his opposition because of a last-minute change that would benefit him personally? Or is it his defense that the provision didn’t influence his support for the legislation because he never actually read the entire bill before changing his position?

The kerfuffle that followed the announcement by Sen. Bob Corker (R-Tenn.) that he would vote for the tax bill when it comes up for a vote this week underscores the unseemly way legislation of such consequence has been cobbled together behind closed doors and with Democrats completely shut out. And the fact that real estate businesses would get a hefty tax break from the provision in question is further evidence of just how much the GOP bill tilts toward the wealthy.

Mr. Corker, as the New York Times reported, surprised many in Washington when he announced Friday that he was dropping his objections to the legislation, thus providing critical support in light of Sen. John McCain’s (R-Ariz.) planned absence. Mr. Corker had voted against the Senate bill because of his concerns over the budget deficit, which it would greatly worsen. So why did he change his mind when nothing was done to address those very real concerns?

Mr. Corker said it was because he felt that on the whole, the country would benefit from the tax overhaul and its expected job growth. But as details about the bill emerged over the weekend, critics suggested his vote was a trade-off for a provision that would make it easier for real estate developers — such as Mr. Corker as well as President Trump — to take advantage of a new, more generous tax structure for pass-through businesses. Mr. Corker denied knowing anything about the benefit and called upon Sen. Orrin G. Hatch (R-Utah) to lay out how it got into the bill. Mr. Hatch offered a convincing explanation that the provision was derived from a House version and was the product of negotiation between House and Senate tax-writing committees that didn’t involve Mr. Corker.

That, though, doesn’t let Mr. Corker, or GOP leadership, off the hook. Mr. Corker — whose vote potentially is key to passage of what will be a transformational remaking of the tax code — changed his position after reading only a short summary and acknowledging that his concerns about the deficit were not alleviated. It is hard not to conclude that it was easier for Mr. Corker to stand on principle when his vote really didn’t matter. As for Republican leaders, it’s hard to be sympathetic to their claims that Democratic partisanship is behind the questions about Mr. Corker’s vote switch. They are the ones who turned their backs on regular order, making a charade of lawmaking and thus inviting suspicion.