After months of promising to rewrite the tax code to bring jobs flooding back from overseas and put more money in the pockets of the middle class, now Congress has to show how it would do that, following the final passage by the House on Thursday of a 2018 budget plan.

The plan sets overall revenue and spending numbers for the federal government but does not provide details of how that revenue will be generated, and the spending details still need to be filled in by House and Senate Appropriations committees.

Next week, the House Ways and Means Committee will unveil tax legislation that shows precisely who falls into what tax brackets; what deductions, credits and "loopholes" are being eliminated or increased; and how the tax system will be transformed for both multinational corporations and small businesses.

The narrow 216-212 vote on the budget indicates that despite near-universal agreement that the tax code needs change, details about what is changed and how that change happens will be critical to passing a bill.

The Republican majority has made it clear that it needs a major accomplishment to show voters in the midterm elections next year.

After passing a budget plan Oct. 5 that required revenue reductions from tax cuts to be offset by closing loopholes or cutting spending, House Republicans reversed field Thursday and accepted a Senate-approved budget allowing for as much as $1.5 trillion in additional debt.

Read more:Conservatives hate overspending. So why did they OK a budget with a $1.5 trillion debt increase?

Many supporters of the tax overhaul say it will spark growth to offset lower tax rates, but deficit hawks such as Sen. Bob Corker, R-Tenn., have said they will oppose a bill that adds too much to the debt.

A key provision of the budget bill is "reconciliation" language that prevents Democrats in the Senate from using a filibuster, which would require 60 votes to overcome, against a tax bill. With the budget's passage, a tax bill would need only 50 votes — when Republicans hold 52 seats — and the support of Vice President Pence to become law.

"This was an enormous step in the direction towards getting comprehensive tax reform and tax cuts for middle-class families over the line, into law, done,” said House Speaker Paul Ryan, R-Wis.

Still, 20 Republicans voted against it Thursday, along with every Democrat in the House. Eleven Republicans from New York and New Jersey opposed the measure as part of their drive to retain a deduction for state and local taxes that Ryan, top aides to President Trump, and an array of conservative political groups have been trying to eliminate.

Eliminating the deduction would provide an estimated $1.3 trillion to offset other tax cuts, and the deduction, referred to by insiders as SALT, has been attacked by critics.

"SALT is bad policy, and Congress should not allow low-tax states to subsidize reckless, big-government tax-and-spend policies in liberal states," said Michael A. Needham, chief executive of Heritage Action, a conservative advocacy group.

But some of those liberal states still have Republicans in Congress. They've joined Democrats who argue their states already pay more in taxes than they get back in federal spending, and eliminating the deduction would amount to a tax increase for many residents, even with lower overall tax rates.

Asked whether passage of a tax bill that eliminated the deduction would be a political death warrant for Republicans from those states, Rep. Tom MacArthur, R-N.J., said that was a "legitimate risk."

"If you want nobody but Democrats to represent these states, then make it impossible for a Republican to win. But again, my focus is not on that, it's on my constituents and making sure they get a fair deal here, " MacArthur said.

Democrats have tried to turn public opinion against the tax plan by pointing to early analyses showing the bulk of the benefits going to people at the top of the income scale. Republicans have countered that the final plan would maintain a balance between the rich and the middle class — which they have struggled to define — while allowing many people making the least to stop paying income tax entirely.

It has been impossible to tell who is right, because of the scant details released so far. That will change Nov. 1, when House Ways and Means Chairman Kevin Brady, R-Texas, says legislation will be released. He said hearings on the tax bill would begin Nov. 6.

"We are ready to deliver tax relief that improves the lives of middle-income Americans and struggling families who have been left behind in our slow-growing economy," Brady said.

Brady told reporters on Wednesday that he expects a deal will be reached to address concerns of lawmakers from high-tax states about the state and local deduction. But preserving that deduction would require generating other revenues elsewhere in the tax code or worsen the impact on the debt, which could cost the tax plan votes from conservatives.

“The minute you start looking at state and local and making concessions there, you’re losing significant revenue dollars that affects, you know, the overall ability to cut rates,” said Rep. Mark Meadows, R-N.C., leader of the influential House Freedom Caucus. “I would prefer to repeal it because it honestly benefits a few states, none of which I represent.”

A Republican tax framework released in September called for shrinking the number of income tax brackets from seven to three, nearly doubling the standard deduction while eliminating personal exemptions and increasing the child tax credit. The plan would eliminate the estate tax, which is only charged on estates worth more than $5 million, and the Alternative Minimum Tax, which is overwhelmingly paid by the wealthy.

Corporations would see the top tax rate drop from 35% to 20%, and "pass-through" businesses — whose owners report their income on personal returns rather than corporate returns — would get a new maximum 25% rate.

But such details as how much the child credit would grow and what income levels would qualify for the new tax brackets of 12%, 25% and 35% have not been revealed. The framework also said it would preserve the deduction for mortgage interest and charitable contributions for those who itemize, but not other deductions. Nevertheless, Congress voted separately to increase a deduction for catastrophic losses for victims of recent hurricanes.

One area where the White House is expecting a fight is over how to write restrictions that prevent wealthy people from classifying what they earn as business income to get the top rate of 25% for pass-throughs instead of the 35% rate for individuals.

Ryan also said last week the House plan would include a fourth rate that was higher than 35% to ensure the wealthiest paid their fair share, though he did not say what that rate would be or what income levels it would affect.

Tax breaks for retirement savings are another wildcard. After published reports said Congress was looking at capping the benefits of putting money into 401(k) accounts, Trump said on Twitter that 401(k)s would not be touched.

But Brady indicated that was still not settled.

"We think in tax reform we can create incentives for Americans to save more and save sooner," Brady said. "So we are exploring a number of ideas in those areas and we’ve invited those who are really experts in retirement savings to come to the table. They’ve brought us ideas on how best to do that. So we’re having that discussion."

Contributing: Eliza Collins