Vice President Kamala Harris is turning to tax policy as a way to differentiate herself from both her Republican rival and her Democratic boss, an attempt to simultaneously project a more moderate image and stand by the soak-the-rich populism that animates much of her party.

During a campaign stop in New Hampshire on Wednesday, Ms. Harris for the first time explicitly said she would scale back one of President Biden’s policy proposals. Instead of taxing the investment earnings of Americans making more than $1 million a year at a 39.6 percent rate, as Mr. Biden has suggested, she called for 28 percent, though a surtax would apply on top of that.

Raising taxes on the rich has long been a central talking point for Mr. Biden and other Democrats, and while Ms. Harris has said she would increase taxes on the wealthy, the capital-gains adjustment was a message to investors and corporate executives: Ms. Harris is a different type of Democratic nominee.

Mr. Biden, who defeated progressives like Senator Bernie Sanders of Vermont in the 2020 primary, still embraced some of their ideas for taxing the wealthy. The plans largely targeted the investment earnings of the very rich, including a so-called billionaire minimum tax that would require Americans worth at least $100 million to pay taxes on unrealized gains of their stocks, bonds and other assets.

Ms. Harris, however, did not have to win a crowded primary campaign in which progressive policy proposals proliferated. She was instead suddenly thrust to the top of the ticket in a general election that Democrats across the party’s ideological wings believe must be won, whatever policy rhetoric necessary. With that opening, some donors have pushed her to redefine the party’s tax platform and abandon Mr. Biden’s tax ideas, like the billionaire minimum tax, adopted when the progressive faction of the party enjoyed more sway.

“My plan will make our tax code more fair, while also prioritizing investments and innovation,” Ms. Harris said on Wednesday. “So let us be clear: Billionaires and big corporations must pay their fair share in tax.”