President Trump’s 2017 tax cuts reduced the top rate on labor income from 39.6 percent, but they did not change capital gains rates. Mr. Trump has considered reducing taxes on certain investment income as part of a still-in-progress package of tax proposals for his re-election bid.

Mr. Biden would raise the top income and capital gains rates to 43.4 percent, for Americans earning $1 million or more annually.

Mr. Sanders would raise capital gains rates in steps for Americans earning $250,000 a year or more. The rate would be 40 percent for Americans earning between $250,000 and $500,000. For those earning above $10 million, the top rate would be nearly 60 percent. (Ms. Warren had suggested setting a top capital gains rate of nearly 60 percent for individual taxpayers earning more than $250,000 a year.)

Both candidates would also end a provision that reduces taxes for the heirs of very wealthy Americans who are subject to the estate tax upon death. If a rich man buys a vacation home for $1 million, but its value grows over decades to $11 million, he typically owes capital gains taxes on the profit of $10 million if he ever sells the home. But if he instead passes the home onto his daughter and she sells the home for $11 million, she owes no taxes at all because the home’s value is reset for tax purposes, or stepped up, to what it is worth at the time of the father’s death.

That provision effectively allows wealthy families to avoid some taxes by holding assets — which are not taxed until they are sold — until they are eligible for the “step up” in value.

Mr. Wyden calls the step-up benefit, and the fact that assets are taxed only when they are sold and not as they appreciate in value, “loopholes” that wealthy Americans exploit to reduce their tax bills.

“These folks to a great extent just get to pay what they want, when they want to, and often hardly anything at all,” he said in an interview. In meetings with constituents, he added, they tell him: “When you talk about new taxes, you should make sure people pay what they owe. Close the loopholes first.”