When Republicans muscled the Tax Cut and Jobs Act through Congress in late 2017, all Democrats could do was howl from the sidelines. Procedurally, Republicans didn’t need Democrats’ support to approve the sweeping tax overhaul for corporations and individuals. When Democrats take the House majority in January, they will use their new investigative powers to conduct oversight hearings of the tax cut law. But Democrats won’t have the power to overturn it. Here are four things to know about how the 2018 elections will impact tax policy in 2019.

1. Democrats unlikely to get Trump’s tax returns, at least not quickly. The most immediate tax consequence of Democrats winning the House majority will be that they will trigger a legal showdown over access to President Donald Trump’s tax returns.

Rep. Richard Neal, D-Mass.

A 1924 law allows the chairmen of Congress’ tax-writing committees to request access to any person’s tax return, including a president’s. Rep. Richard Neal, D-Mass., the incoming chairman of the Ways and Means Committee, said he would ask for Trump’s tax returns.

Many congressional Democrats remain angry that Trump refused to release his federal tax returns as other presidential candidates had by tradition done – even though not required by law – for several decades. The returns may include information about Trump’s business dealings that could be relevant to federal investigations into Russian efforts to influence the 2016 elections and other issues, Democrats say. Trump said he’s resisting release his returns because he’s under an Internal Revenue Service audit.

Under the law, Neal would make his request to Treasury Secretary Steven Mnuchin. Although the law says the Treasury secretary “shall furnish” any tax return properly requested, Mnuchin would presumably refuse, triggering a lawsuit against the Trump administration by House Democrats.

The issue may not be decided for years.

2. Tax Cuts and Jobs Act. Neal said the committee will hold hearings on the $1.5 trillion Tax Cut and Jobs Act, including how specific provisions are benefiting specific stakeholders.

But what Neal won’t be able to do is change the underlying tax law – either by repealing it or even substantially changing it.

House Democrats may take up legislation rolling back parts of the Tax Cuts and Jobs Act. For example, they may vote to reinstitute the full state and local tax deduction that the law scaled back. But any legislation effectively raising taxes would be stalled in the Republican-controlled Senate.

Trump, in a post-election news conference, said he could support reversing some of the tax relief included in the 2017 law in exchange for other tax cuts. “If the Democrats come up with an idea for tax cuts, which I’m a big believer in tax cuts, I would absolutely pursue something even if it means some adjustment [to the Tax Cut and Jobs Act],” Trump said.

Neal said any new tax cuts would need to include increases in the top individual marginal rate. The incoming chairman also is unlikely to advance legislation sought by Republicans making technical corrections to the tax reform law, wanting first to focus on his committee’s hearings that he hopes will reveal the tax law’s shortcomings.

3. Democrats’ tax agenda. While the Ways and Means Committee is expected to focus on oversight, it also will need to develop its policy agenda. Key to that is balancing the desires of Democrats’ liberal wing, who want to try to repeal the Republican-backed tax cut law, with newly elected moderate Democrats who may only want to advance narrow changes.

Even legislation that seeks to send a message or sharpen political definitions may prove difficult to pass in the House, where Democrats’ majority is slimmer than the Republican majority it is about to replace. Many newly elected Democrats are from tossup or lean-Republican districts, and those lawmakers may not want to be on record voting to raise taxes – especially if the bill cannot pass the Republican-controlled Senate.

It’s possible there are tax bills the two parties could work on and pass out of both the House and Senate, including updating tax incentives for retirement and other savings accounts.

But the tax agenda in 2019 will mostly be focused on Trump’s tax returns and Democrats’ oversight of the tax reform law.

Sen. Chuck Grassley, R-Iowa

4. Senate Finance Committee. The Senate Finance Committee will get new leadership in 2019 after the retirement this year of Chairman Orrin Hatch, R-Utah. The panel is the principal Senate committee with jurisdiction over both taxes and trade.

Sen. Chuck Grassley, R-Iowa, will most likely become the next chairman. He would have to relinquish the chairmanship of the Judiciary Committee, which under his leadership successfully confirmed two Trump-appointed Supreme Court justices and a record number of federal appeals court judges.

If Grassley elects to stay at the Judiciary Committee, Sen. Mike Crapo, R-Idaho, would be next in line to become chairman.