The Trump administration is considering bypassing Congress to grant a $100 billion tax cut mainly to the wealthy, a legally tenuous maneuver that would cut capital gains taxation and fulfill a long-held ambition of many investors and conservatives.Steven Mnuchin, the Treasury secretary, said in an interview on the sidelines of the Group of 20 summit meeting in Argentina this month that his department was studying whether it could use its regulatory powers to allow Americans to account for inflation in determining capital gains tax liabilities.

As a candidate, Mr. Trump declared that he understood America’s complex tax laws “better than anyone who has ever run for president” and that he alone could fix them. But it is becoming increasingly unlikely that there will be a simpler system, or even lower tax rates, this time next year. The Trump administration’s tax plan, promised in February, has yet to materialize; a House Republican plan has bogged down, taking as much fire from conservatives as liberals; and on Monday, Treasury Secretary Steven Mnuchin told The Financial Times that the administration’s goal of getting a tax plan signed by August was “not realistic at this point.”