There's just one problem: although individual income tax rates have gone down since 2017, individual income tax revenues have gone up.

According to the White House budget office, Federal Tax revenues were as follows:

(This does ignore Social Security and other trust funds.)

But how is this possible?  How could tax rates have gone down, but taxes collected gone up?

Think of the government as, say, a Cadillac dealer.  If prices for Cadillac cars go up, it's not necessarily true that revenues from selling Cadillacs go up, too.  It depends on a lot of other things — prices and availability of other luxury cars and discretionary funds available to car-buyers.

With income tax rates down, a doctor might decide to work more hours, feeling he will be keeping more of the extra fees coming in rather than handing them over to Uncle Sam.  A farmer may choose to plant more crops.

One type of tax collections did fall — monies collected from American corporations.

Was it really money thrown away?

It was the cost for jump-starting the economy, expanding wages and employment.  It made America a more attractive location for building factories and operating businesses.  It brought unemployment rates for black, Hispanic, and Asian-Americans to historically low levels.

Is this really what the Democrats want to reverse?

Kamala Harris wants tax rates going back up.  CNBC reported:

Sen. Kamala Harris of California wants to give $2.8 trillion in federal tax credits to lower- and middle-class families. Her plan provides a direct counter to the Republican proposal, under which the $1.5 trillion in cuts skewed toward companies and wealthier Americans. She wants to pay for the measure in part by reversing the pieces of the GOP overhaul that "benefit the rich," as well as by putting "a new tax on large financial institutions," according to The Washington Post.

Joe Biden is also in favor of reversing the cuts.  Americans for Tax Reform reported (video included):

Biden recently told a crowd in South Carolina that it would be the very first thing he does upon taking office: "First thing I'd do is repeal those Trump tax cuts," he said.

CNBC sums it up:

Various other Democratic candidates including Sen. Elizabeth Warren of Massachusetts and Sen. Bernie Sanders of Vermont want to roll back the GOP tax cuts and implement new taxes on the wealthy to fund social programs such as universal child care and "Medicare for All."

It is true that much of the benefit from Trump's tax cuts went to the nation's wealthiest.  Duh.  That's because it's the wealthiest who pay the bulk of our taxes.

An October 2018 Bloomberg analysis concluded that the bottom half of American taxpayers contributed only 3% of income taxes paid.  The top 1% of earners accounted for more than 37% of income taxes collected.

A summary of the 2017 tax cuts is here.

Biden, Harris, Sanders, et al. may want to re-read Aesop's Fables — in particular, the one about a goose that laid golden eggs.