As part of his closing argument ahead of the Nov. 6 midterm elections, President Trump has been promising that Congress is about to cut taxes for the middle class.

It isn’t.

Here he was in a rally Monday night in Texas, detailing a cut that no one on Capitol Hill or K Street was talking about until a few days ago:

We’re going to be putting in a 10 percent tax cut for middle-income families. It’s going to be put in next week, 10 percent tax cut. Kevin Brady is working on it. We’ve been working on it for a few months, a 10 percent brand-new — and that is in addition to the big tax cuts that you’ve already gotten. But this one is for middle income. This is — no business. Business is now good. They’re coming back. The jobs are coming back. The plants and factories are coming back like never before. They’re all coming back. This is for middle-income people, all middle-income people, a big tax, 10 percent. We’ll be putting it in next week.

Representative Kevin Brady of Texas, the Republican chairman of the House Ways and Means Committee, was the chief architect of the $1.5 trillion tax cut Congress passed last year. His office has provided no detail about what Mr. Trump might be talking about, though Mr. Brady said late Tuesday that Republicans “will continue to work with the White House and Treasury over the coming weeks to develop an additional 10 percent tax cut focused specifically on middle-class families and workers, to be advanced as Republicans retain the House and Senate.”

A spokeswoman for Speaker Paul D. Ryan, the Wisconsin Republican who helped push through last year’s tax cut, referred calls to the White House.

Mr. Brady’s comments suggest that Mr. Trump’s proposed tax cut could be — at best — a blueprint for what Republicans might do if they keep control of the House and Senate next year. It is clearly an applause line at rallies. It has brought tax cuts, as a topic, back onto cable news and given Mr. Trump a way to talk about last year’s tax cut, which has been somewhat of a dud on the campaign trail.