The policy, which Ms Harris is expected to maintain, would only apply to Americans with assets worth more than $100 million.
But high net worth individuals argue that taxing capital gains before they have been realised could force them to sell off assets prematurely to pay the tax bill.
Ms Harris’s focus on capital gains and tax “loopholes” for the wealthiest mirrors plans by Labour to increase the same taxes in the UK.
Rachel Reeves, the Chancellor, has ruled out taxes on “working people”, but signalled that she intends to target investors and property owners by increasing capital gains tax.
Like Ms Harris, the UK Government believes that taxing investment income at a lower rate than income from work is an unfair advantage enjoyed by the wealthy.
If Ms Harris scraps the higher capital gains rate proposed by Mr Biden, she will reduce the planned $5 trillion revenue that could be spent on her other priorities.
She has proposed increases in child tax credits and income tax credits, and pledged to set up a new scheme to help first-time buyers with deposits.
Her other plans include a federal ban on “price gouging” by grocery retailers, which Donald Trump has compared to Soviet-era price controls.
On Wednesday afternoon, she is set to unveil new policies to support American small businesses.