Someone asked me a very simple question Monday night: What's the biggest “pay for” in the Republican tax plan?

It's a key question for Thursday when we get to see the plan that President Trump and congressional Republicans have been working on for months. As businesses big and small cheer lower tax rates, who will pay more to make those changes happen?

Based on what we know right now, the biggest pay for is debt.

American taxpayers of tomorrow will pay a substantial portion of the bill for lower corporate taxes today. Republicans, who bemoaned America's $20 trillion debt for years, have now approved adding $1.5 trillion more to the national debt to “pay for” (i.e. borrow for) tax cuts.

“This move is a sharp reversal from previous Republican promises of revenue-neutral tax reform,” wrote the nonpartisan Committee for a Responsible Federal Budget in a memo Friday.

Here's the basic math. CFRB estimates the total cost for the entire tax package is $5.8 trillion over 10 years, meaning about a quarter of the bill will come from debt financing. The rest is supposed to come from someone (or some business) paying more, but so far, Republicans have struggled to identify who that will be.

Republicans need more than $4 trillion in revenue raisers over a decade. In September, they outlined measures that could raise up to $3.6 trillion, but many of those ideas aren't going over well with the public — or even some GOP lawmakers. The tax writers in the House Ways and Means Committee huddled late in the night on Halloween to try to find some tax increases they can agree on. Many fear the final bill will fall substantially short of paying for itself without accounting gimmicks.

“The bottom line, it seems, is that the big tax cut that the administration would like can’t be sold with only the money-back guarantee that it will pay for itself. So the administration has to raise taxes to cut taxes,” wrote economist Ed Yardeni, president of Yardeni Research, in his morning note Wednesday.

The general principle of tax reform is that the government lowers the tax rates, but it also “broadens the base” at the same time. Instead, this bill is looking more and more like a traditional tax cut.

There's a lot of nostalgia for the 1986 tax reform under President Reagan. The final bill — which nearly died many times over political infighting — lowered rates on individual American taxpayers. It also lowered corporate rates, but the entire bill was fully paid for because politicians did the hard work of eliminating a lot of corporate tax deductions. Alan Auerbach, director of the Burch Center for Tax Policy and Public Finance at the University of California at Berkeley, refers to the 1986 bill as a tax hike on corporations, not a cut.

In 1986, corporations basically footed the bill for individual tax cuts. It's shaping up to be the reverse this time around.

The biggest pay for in the plan was supposed to be eliminating deductions, especially the state and local tax deduction (“SALT”) that is popular in high-tax (and mostly Democratic) states like New York, California and New Jersey. Eliminating SALT would raise $1.3 trillion over a decade, a sizable amount. But Republican members of Congress from many coastal states have threatened to revolt, and the GOP can't afford to lose more than 23 GOP votes in the House and two in the Senate.

Another “pay for” is getting rid of the personal exemption and the exemption for children (or dependents). But it's hard to call this a true pay for since Republicans use much of the money raised from repealing this exemption to pay for a doubling of the standard deduction. The net effect is less than $1 trillion in savings.

If you are keeping track, the math isn't adding up to $4 trillion. It's why the GOP has been trying new ideas. Last week Rep. Kevin Brady (R-Tex.), chair of the House Ways and Means Committee that writes the tax bills, said he was considering changes to the popular 401(k) tax break. Brady didn't lay out any specifics, but one option GOP leaders were reportedly considering would cut the annual amount Americans can put in a 401(k) tax-free from $18,000 to $2,400. The backlash was quick. More than 63 million Americans have an active 401(k) plan. People were upset and so was the financial industry. Trump himself weighed in, publicly promising there would be no 401(k)-related changes.

Among the easiest ways to generate some much needed cash for the bill would be to tax the rich more (or to not lower their taxes in the first place). It would enable the GOP to do the corporate rate cutting they want and also give the middle class a little something extra. It might even win some Democratic votes for the final package.

Not repealing the estate tax, which only estates worth over $4.59 million pay, would save $269 billion over a decade, according to Joint Committee on Taxation estimates.

Not cutting the top tax rate of 39.6 percent, which applies to individuals making more $418,400 a year (and married couples making over $470,700), would save about $265 billion over a decade, according to Tax Policy Center calculations.

Republicans are finally looking at ways to tax the rich. As The Washington Post reported Tuesday evening, Republicans now plan on keeping the 39.6 percent rate in place, at least on families earning at least $750,000 to $1 million. It's a high bar, but it's a sign “tax cuts for everyone” just don't add up.

The question is who's paying for the rest of the tab?