Joe Biden tells us he is intent on winning in November “for the workers who keep this country going, not just the privileged few at the top.”

The election is a referendum not only on the moral failings of President Trump, Democrats argue, but on the economic fissures of the new economy. It is a fight, Mr. Biden says, on behalf of “the young people who have known only an America of rising inequity and shrinking opportunity.”

Why on earth, then, are Democrats fighting — and fighting hard — for a $137 billion tax cut for the richest Americans? Mr. Biden, Nancy Pelosi and Charles Schumer don’t agree on everything, but on this specific issue they speak with one voice: the $10,000 cap on deductions for state and local tax (better known as the SALT deduction) must go.

The House of Representatives has already passed legislation removing the cap, allowing the amount of the deduction to rise. If the Senate turns blue in November, Democrats have promised to return to the issue. “I want to tell you this,” Senator Schumer said in July, “If I become majority leader, one of the first things I will do is we will eliminate” the SALT cap “forever.” It “will be dead, gone and buried.”