When President Trump adds his distinctive signature to the tax bill, he will also be making a huge bet that the Republican strategy of deep cuts for businesses and wealthy individuals will fuel extraordinary growth across the board.

Perhaps more than any other American political leader, Mr. Trump knows that long shots, like his own presidential bid, sometimes pay off. In that vein, he and congressional Republicans are arguing that their bitterly contested and expensive rewrite of the tax code will ultimately create more jobs and raise wages.

If they are proved correct, they will be repudiating not only historical experience, but most experts. From Congress’s own prognosticators to Wall Street’s virtuosos, scarcely any independent analyses project anything like the rosy forecasts offered by the president’s top economic advisers.

To Mr. Trump and his allies, the normal models just do not fully capture the high-octane “rocket fuel” embedded in the tax plan. Mr. Trump intuitively understands just how much attitudes and expectations can shape economic decisions.