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Republican tax writers in the House and Senate scoured the U.S. tax code this week and shook the couch cushions for loose change, as one member put it, in a struggle to find ways to pay for the deep tax cuts their leaders and President Donald Trump have promised.

By late Thursday, the House Ways and Means Committee had hammered together a bill and sent it toward the House floor for a vote promised next week, while the Senate Finance Committee revealed a proposal that it intends to mark up on Monday.

But there's an elephant in the room. Both plans contain nearly $1.5 trillion in red ink in the first 10 years. Unless they eliminate the red ink beyond that — a tall order that would require major changes — the legislation will be subject to a 60-vote threshold under Senate rules, which could doom it to failure. An alternative is to sunset some of the provisions after a decade, but congressional leaders don't want that.

As written, the Senate proposal "blows a massive hole in the debt," said Maya MacGuineas, president of the nonpartisan Committee for a Responsible Federal Budget.

A GOP aide to the finance panel said everything in the proposal that Senate Finance Chairman Orrin Hatch, R-Utah, released Thursday is designed to be permanent — and that any issues related to deficit effects will be fixed. The aide didn't specify how.

Hatch's plan seeks to shore up revenue by departing from the House bill in a number of ways — including its proposal to fully repeal state and local tax deductions that benefit individuals in high-tax states.

It would lower the price tag by about $100 billion with a one-year delay on the corporate tax cut that Trump has sold as a key to faster economic growth. White House Budget Director Mick Mulvaney downplayed the harm of a delay Thursday. But a day earlier, Treasury Secretary Steven Mnuchin said, "The longer we wait, the worse it is for the economy and making companies competitive."

The Senate plan preserves the estate tax — though limiting it to fewer multimillion-dollar estates. The House bill would eliminate that levy.

Still, Hatch's plan would give corporations a bigger break on taxes associated with trillions of dollars in earnings that they hold offshore than the House bill. And it would set a lower top individual tax rate than the House — 38.5 percent for million-dollar earners as opposed to 39.6 percent.

The Senate plan seized on a provision that the House dropped. Hatch proposed accelerating the taxation of salary and other awards put into "non-qualified deferred compensation plans," which function as supersized 401(k) plans for high earners.

The provision, which would also change how stock options and similar securities are taxed, would probably lead to hundreds of companies eliminating such plans, experts say. In the Senate plan, the measure is credited with producing $13.4 billion over 10 years, according to Congress' Joint Committee on Taxation.

If those differences sound difficult, just wait. Jonathan Traub, who oversees tax policy for Deloitte Tax, said the biggest sticking points will emerge as the Senate finance panel begins rehabbing its bill to comply with Senate rules.

"That's a hole that Hatch and the other Senate Republicans are going to have to fill," Traub said. "And how they fill it may not be very attractive to House Republicans."

On the House side, last-minute changes to the bill before it cleared the Ways and Means Committee pulled its price tag just under the $1.5 trillion allowed in the first decade under the budget resolution.

"Today, the first and oldest committee in Congress passed transformational tax reform legislation that charts a new course for the country," Ways and Means Chairman Kevin Brady, R-Texas, said after his panel approved his tax bill.

To get there, the panel appears to have employed a touch of fiscal finesse: The single biggest source of new revenue in Brady's changes stems from a revision that wouldn't take effect until 2023 — raising questions about whether Republicans in Congress would ever really let it happen.

The measure would require companies to write off their spending on research and experimentation over five years. Currently, they can deduct such spending immediately. That change raises $108.6 billion from 2023 through 2027.

A wild card in the debate is the Affordable Care Act individual mandate, which many Republicans are pushing to repeal as part of a tax bill. Doing so would save $338 billion in government subsidies over a decade to help offset tax cuts. But it's a political hand-grenade that party leaders worry could cost them crucial votes and sink the tax effort.