The Budget Lab at Yale had interesting proposals on how to accomplish this:
https://budgetlab.yale.edu/research...reatment-borrowing-against-appreciated-assets
Option (1): Deemed Realization for Borrowing
This option, based on a proposal by Liscow and Fox (2024), would treat loan proceeds as a “deemed realization” of unrealized capital gains. That is, borrowing would be treated as a realization event. After tax is paid, the basis of the affected assets would reset to fair market value, preventing double taxation if the assets are later sold.
Option (2): Withholding Tax on Borrowing
This option would apply a simple flat-rate “withholding” tax on loan proceeds, with the withheld amount creditable against future capital gains tax liability. Under the proposed 10% withholding rate, a $10 million loan would trigger an immediate $1 million tax payment regardless of the taxpayer's basis in their assets.
Option (3): Annual Excise Tax on Loan Balances
This reform, based in part on a Bipartisan Policy Center proposal, would impose a 0.5% annual tax on the outstanding balance of covered loans.1 In contrast with the prior two reform options, which would apply to net new flows of borrowing, this option would apply to the stock of debt. For example, a $10 million loan balance would incur an annual liability of $50,000.