“The sun may be shining outside, but today is a gloomy day here in the United States Senate,” the chamber’s majority leader, New York Sen. Chuck Schumer, said from the floor on Tuesday.

Schumer was, in the immediate sense, referring to the status of the so-called national security supplementary package that members of both parties had written together—a bill that would provide military aid to Ukraine and Israel, fund the hiring of new border security personnel, and give the executive branch more discretion to turn away migrants seeking asylum in the U.S. The bill looks unlikely to pass, as Schumer noted.

But he was also referring more broadly to what its impending failure says about the existential condition of the Senate. As Schumer observed, the most noteworthy pieces of the supplemental bill are its immigration-related provisions, which had been “fervently demanded” by the GOP in November and were added to the legislation, over the course of two-plus months, by Republican Oklahoma Sen. James Lankford, who had been deputized by Minority Leader Mitch McConnell. Republican senators like........

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