Many provisions of the 2017 Tax Cuts and Jobs Act, one of the signature achievements of the first Donald Trump administration, are set to expire in the coming year, and Republicans are keen to use their trifecta of political control to determine what tax policy will look like in the second Trump administration and beyond.

But even though one party will control both houses of Congress and the White House, there is still plenty for lawmakers to argue about.

The Tax Cuts and Jobs Act changed what most American people and businesses owe the IRS every year. The law increased the standard deduction, so fewer people itemize on their tax returns, and it cut the tax rate for corporations and most private businesses.

But there was a catch. In order to lower the overall cost of the law, at least on paper, Congress used some accounting and legislative “gimmicks,” according to Owen Zidar, an economics professor at Princeton University.

“The way that the original 2017 bill got passed was they made some of the things expire, in particular, the things that they thought might be hard to have people stop, like tax cuts for a broad group of people,” Zidar said.

Most of the tax cuts for individuals and some of the provisions for businesses expire at the end of 2025. The new Congress that takes office in January will have to decide which tax changes to extend, whether to add new components to the law and how to pay for them.

“A lot of the tension will be about different groups within the Republican Party,” Zidar said. “Some of them care a lot more about economic growth. Some of them care about a range of other things.”

For example, what to do about the state and local tax deduction, or SALT cap, which was lowered to $10,000 in the 2017 law? Before that, people could deduct what they paid in state and local taxes from their federal income taxes.

“A number of Republicans, particularly those in higher-tax states like New Jersey, New York, California — the sort of the states that were most affected by the SALT cap — want to raise the SALT cap or lift it,” said Samantha Jacoby, deputy director of federal tax policy at the Center on Budget and Policy Priorities.

Either option would increase the cost of new tax legislation, in addition to the potential cost of tax-related changes Trump promised on the campaign trail. 

“Exempting tipped income from income tax, exempting Social Security benefits, benefits from income tax, providing a special deduction for auto loan interest on your tax return that you pay. All of those, of course, are going to have revenue effects,” said Garrett Watson, a senior policy analyst at the Tax Foundation.

The cost of all of these changes together would run into trillions of dollars over 10 years, the period of time by which most pieces of legislation are judged in terms of cost. Just renewing the expiring provisions of the 2017 law alone — most of which mostly benefited wealthier Americans — would cost an estimated $4 to $5 trillion over a decade.

To cover the final bill, whatever it turns out to be, Republicans in Congress are floating a variety of options.

“Republicans are starting to threaten big spending cuts to programs like Medicaid and food assistance to either pay or offset some of the tax cuts, or basically just to lower the size of government, because our tax revenues are not enough to keep up,” said Jacoby of the Center on Budget and Policy Priorities.

“And of course, [Trump]’s also considering tariffs as one potential offset or revenue raiser for some of these other tax cuts. So, lots of moving parts here, a lot of uncertainty,” Watson said.

Although Watson, along with many other economists, question how — or if — tariffs funding tax cuts would even work, yet alone raise enough revenue to offset extending or adding to the tax cuts. 

Which leads to one of the big-picture debates already percolating among Republicans in Congress: Just how much they are willing to add to the deficit to get all of this done?

“Some folks talk about fully offsetting the cost of extending these tax cuts,” said Watson. “So, in that case, the deficit impact might be zero, all the way to let’s fully finance this by increasing the deficit. And that number could be north of $5 trillion over 10 years. And so that’s the range we’re talking about.”

Complaining about the growing deficit was one thing when Republicans were in the minority or in divided government, but could be another thing entirely now that they have unified control of the White House and Congress. Especially as any tax policy creates winners and losers, with all of their affected constituencies and their lobbyists clamoring for their own special tax breaks.