There is also a question of politics: Democrats have already assailed Mr. Trump’s $1.5 trillion tax cut as a giveaway to the wealthy. Some Republicans, including senior Treasury Department officials, have privately expressed worry that a unilateral tax cut for investors would simply give Mr. Trump’s potential 2020 rivals more fodder for those attacks.

The plan under discussion would require the Treasury Department to change the definition of “cost” for calculating capital gains, allowing taxpayers to adjust the initial value of an asset, such as a home or a share of stock, for inflation when it sells. Such a move would lower investors’ tax bills by effectively reducing the profit earned on the sale of their assets.

Brian Callanan, who is the department’s deputy general counsel and has been nominated as its next general counsel, told staff members on the Senate Finance Committee before his confirmation hearing last week that the change could not be made without a new Justice Department memorandum declaring it legal.

“In a meeting prior to his hearing,” said Ashley Schapitl, a spokeswoman for Democrats on the committee, “Mr. Callanan indicated to Finance Committee staff that he would adhere to the Office of Legal Counsel’s 1992 opinion that the Treasury Department could not unilaterally index capital gains rates to inflation. If the Treasury general counsel nominee doesn’t believe the department has authority to unilaterally give the top 1 percent nearly $100 billion in additional tax cuts, that should put the issue to bed.”

The Justice Department did not immediately respond to a request for comment.

Capital gains come from the sale of an asset, like a stock or a house, that has grown in value over time. Gains from assets held by a taxpayer for more than a year are taxed at a maximum rate of 20 percent, which is lower than the top rate on income earned as wages. Currently, the gains are calculated using dollar amounts that are not adjusted for inflation: If an investor bought a tech stock for $10 in 1980 and sold it for $50 today, she would owe tax on a $40 capital gain.