JERUSALEM — The 2017 Republican tax cuts have been a dud on the campaign trail ahead of the November midterm elections, so President Trump has come up with a new plan: more tax cuts.

In Nevada on Saturday, Mr. Trump said he and Republican lawmakers had been working on “a very major tax cut” for middle-income people that would be rolled out in the coming weeks.

There is no chance of such a plan passing even the House before the midterms, let alone the Senate, because Congress is in recess through the election. So the move appears to be an effort to give Republican voters a jolt of enthusiasm as the polls are opening — and perhaps an acknowledgment of how small a boost Mr. Trump’s signature tax bill seems to be giving Republicans in the battle to control Congress.

Steven Mnuchin, the Treasury secretary, said in an interview on Sunday that he had been working diligently with Representative Kevin Brady of Texas, the Republican chairman of the House Ways and Means Committee, to develop another tax plan that would be released “shortly.”