Republicans are accelerating efforts to fill in key details of their plan for massive tax cuts, but as lawmakers work to turn their proposal into legislation, President Trump’s numerous tax promises are proving difficult to keep.

On Monday, Trump promised the party would not touch tax benefits for 401(k) retirement plans, protecting a popular benefit for more than 50 million Americans but also further limiting the areas where Republicans could seek to raise new revenue.

His vow to protect 401(k) plans, made in a Twitter post, comes just days before House Republicans are planning to introduce a bill that would dramatically slash corporate tax rates, consolidate tax brackets for families and individuals, and eliminate the alternative minimum tax and estate tax.

Rep. Kevin Brady (R-Tex.), who chairs the House Ways and Means Committee and is leading initial drafting of the tax legislation, is expected to release the bill as soon as next week and told reporters it remains on track. But he declined to make a firm commitment on timing and said Republicans had not yet made key decisions, such as whether to add an additional income tax bracket for high earners and on how to implement a planned estate tax repeal.

“I want to get this to the president’s desk by the end of the year, and we’re on a schedule to do that,” Brady said.

Rep. Kevin Brady R-Tex.) is leading initial drafting of the tax legislation. (Andrew Harrer/Bloomberg)

The remaining indecision shows the competing pressures tax writers face as they attempt to deliver the massive tax cuts Trump has promised while also minimizing the effect of the tax cuts on the deficit.

The challenge has become particularly acute now that the Senate has passed its budget resolution, which the House of Representatives is expected to approve by Thursday. This procedural step allows them to eventually pass a tax bill in the Senate with just 50 votes, instead of the 60 typically required to advance major bills.

They must now write legislation that can pass the House and Senate, something they still haven’t done despite dozens of meetings in the White House and on Capitol Hill. They remain unable to reconcile all of Trump’s assurances that there will be massive tax cuts benefiting everyone with Senate rules that limit how much they can do.

House Republicans hope to introduce a bill as soon as next week, but they are still working through key details that could derail the bill if mishandled. Still, GOP congressional aides on Monday expressed confidence that their bill was coming together rapidly.

The budget resolutions allow the tax bills to add $1.5 trillion to the deficit over 10 years, a large amount nonetheless far outstripped by the more than $5 trillion in promised tax changes Trump has laid out so far.

Now Republican negotiators are looking at cutting back on many of the deductions families and businesses use to limit their taxes to offset the lower rates, but Republicans and the White House are under pressure to back away from some of the demands.

One of the changes Republicans had floated was simplifying the tax benefits tied to retirement benefits, fueling concerns that they could make changes to 401(k) plans. Americans are allowed to contribute up to $18,000 pretax into their 401(k) plans each year as a way to incentivize saving for retirement. Lowering that pretax threshold could raise more revenue but face a backlash from many Americans who use the accounts to save for retirement.

After several news reports in recent days saying the changes were possible, Trump weighed in on Monday and said no changes would occur. Now the tax writers must look for other areas where they can raise money, and their options appear to be dwindling.

“You are trying to stuff a $4 trillion or $5 trillion tax cut in a $1.5 trillion box,” said Steve Moore, who was one of President Trump’s top economic advisers during the 2016 campaign. “That means something has to give here.”

Senate rules will prevent Republicans from passing a tax bill with a simple majority if it adds to the deficit after 2027. Republicans have already promised not to jettison Americans’ ability to deduct their mortgage interest, charitable contributions, and now income for 401(k) contributions, limiting the number of other changes they could make to raise revenue.

Party leaders now believe they will only be able to make some of the tax cut changes permanent, and others temporary, expiring after 10 or fewer years.

Business groups are lobbying Republicans to ensure that the corporate tax cuts are permanent, but this could put Republicans in an awkward position of promising companies better tax treatment than families, particularly as Trump has said the plan will primarily be a middle-class tax cut.

“They have probably overpromised and are trying to find a way to fit the plan into whatever constraints they have,” said Mark Mazur, director of the Tax Policy Center, who was a senior official in the Treasury Department during the Obama administration.

And there are numerous parts of the tax package that will be difficult to enforce, leading to concerns that wealthy individuals could try to game the changes to further lessen their tax bill.

Chief among the problems is finding a way to prevent upper-income households from taking advantage of Trump’s vow to slash the rate certain businesses pay.

Lawmakers are concerned the wealthy could take advantage of this change to lower their taxes, even though the rate is supposed to apply to businesses, not individuals. Republicans haven’t settled on a way to prevent such abuse even though they’ve known it was a problem for months, though competing House and Senate proposals could emerge within days.

Negotiators have run into huge problems sorting out several of the White House’s promises.

The biggest one is Trump’s insistence on lowering the rate for millions of businesses that pay taxes through the individual income side of the code. Many of these companies now face a top rate of 39.6 percent, and Trump has promised to lower it to 25 percent. But negotiators fear that upper-income people could try to reclassify themselves as a certain type of corporation to pay a lower tax rate than typical households pay.

The White House has not provided any estimates for how it would enforce this or prevent people from improperly taking advantage of the changes. The Committee for a Responsible Federal Budget, a nonpartisan think tank that advocates for deficit reduction, has estimated lowering the rate for these companies would lead the government to lose approximately $500 billion in revenue over 10 years.

The GOP tax plan calls for collapsing the seven income tax brackets individuals and families now pay down to three brackets of 12 percent, 25 percent and 35 percent. But congressional Republicans and the White House continued debating whether they should add an even higher rate for upper-income Americans as a way to ensure the wealthy don’t disproportionately benefit from the plan. They have not decided how to proceed and are waiting for other details of the plan to come together before they decide whether to pursue the higher rate.

Even with a higher rate, there are numerous other provisions of the package that are expected to disproportionately benefit the wealthy, and they would still pay a lower tax rate on all of their income up to whatever new limit is put in place.

The White House and lawmakers are also trying to decide how to treat income U.S. companies earn abroad. Currently, companies must pay taxes on that income, but they can defer paying if they keep the earnings overseas. Trump has said companies keep more than $5 trillion in earnings overseas in order to avoid paying taxes, and he wants that money brought back to the United States.

To remove the incentive from earning profits overseas, the White House and Republicans are considering the imposition of a foreign “minimum tax,” which could require companies to pay the U.S. government a portion of their global income, regardless of where it is earned.

Presently, companies can indefinitely defer paying taxes on income earned overseas, so long as they don’t try to repatriate those earnings back to the U.S. parent company.

This could have a major impact on multinational firms, particularly technology and pharmaceutical companies, which are organized in such a way that a lot of their income is channeled through overseas subsidiaries in low-tax countries such as Ireland. It is unclear what the rate might be or whether the taxation will be applied differently if income is earned in different countries.

Similarly, the White House and Republicans have not decided whether to impose any restrictions on income that multinational companies bring back to the United States at a lower tax rate.

In 2004, the Bush administration and Congress agreed to a temporary “tax holiday” that allowed companies to bring foreign earnings back to the U.S. at a low rate, but studies have found that this money did not go toward hiring and new investment. Rather, some of the companies that benefited from the repatriated earnings proceeded to cut jobs, raise dividends, and boost executive compensation.

Some Republican negotiators are considering whether to add some kinds of restrictions to this part of the tax package that requires companies to reinvest some of the earnings back in a way that grows U.S. jobs or wages, people familiar with the planning said.

Despite the lingering differences, Trump administration officials and congressional aides said that months of negotiations, particularly in the past few weeks, have allowed them to make progress in a number of areas.

For example, they believe they have largely addressed complaints raised by Republicans in states like New York, California, New Jersey and Illinois. The lawmakers had complained that taxes for people in their states might actually go up if the new plan prohibits people from deducting their state and local taxes from their taxable federal income. After a number of meetings with concerned lawmakers, GOP leaders think they have found a way to minimize the impact of middle-class residents in these states, though precise details of how this would work could not be learned.

There are a number of other decisions that still need to be made. Negotiators have yet to detail what income levels will qualify for each tax bracket and how an expanded child tax credit will work. These are details that will be necessary for them to try to convince voters of the potential benefits for the middle class, as much of the discussion so far has focused on how businesses will be able to pay less in taxes.

After spending months insisting that the tax-cut plan would not benefit the wealthy, the White House is now giving conflicting assessments of whether this would be the case. Trump has said repeatedly that the wealthy have nothing to gain from the tax changes, but Treasury Secretary Steven Mnuchin said otherwise in a recent interview with Politico.

“When you’re cutting taxes across the board, it’s very hard not to give tax cuts to the wealthy with tax cuts to the middle class,” Mnuchin told Politico last week. “The math, given how much you are collecting, is just hard to do.”

Brady is holding meetings with Republican colleagues this week as they try to narrow differences and create their first version of the tax bill, which they hope will pass the House next month.

If it passes, the Senate would have to deliberate on their own version through a similar process. Lawmakers are hopeful they can complete the votes by the end of December, but they will need to sort through all of these decisions before then.

“There is a lot of desperation on the part of Republicans, and desperation is what will get us over the hump ultimately,” Moore said.