The Senate is emerging as a possible hurdle to achieving tax reform this year, as Senate Republicans have failed to say which tax breaks they're willing to take away in order to pay for tax rate reductions, unlike House Republicans who have already offered some ideas.

If Congress doesn't eliminate big tax breaks, it won't be able to lower tax rates without blowing out the federal deficit. Republicans have set ambitious goals for tax rates, with President Trump calling to lower the corporate tax rate from 35 percent to 15 percent and House Republicans setting a target of 20 percent.

In their tax reform blueprint, House Speaker Paul Ryan and House Republicans identified three major ways they would raise revenue: One would be eliminating all itemized deductions, except for those for mortgage interest and charitable giving; another would be to prevent businesses from deducting the cost of interest on loans; and last would be the border adjustment.

While border adjustment has stirred the most recent controversy, all three would be difficult to pass. While none has a strong advocate in the Senate, all three have critics.

"That's obviously one of the problems you have with any kind of tax reform, is everybody loves their tax preference," said Sen. Ron Johnson, R-Wis.

Senate Finance Committee Chairman Orrin Hatch has suggested that all three of the House's major revenue-raisers, or "pay-fors," do not enjoy support in the Senate at the moment.

"I don't think in the final analysis they'll be able to do that," Hatch said of the idea of limiting the ability of businesses to deduct interest payments from taxable income, speaking after a meeting with the White House and House on tax reform this month.

Like every tax break, the deductibility of business interest payments has a dedicated coalition of businesses defending it. Real estate investors, private equity funds, telecommunication companies and other groups that have debt built into their business models will lobby to keep it in the tax code.

In theory, Republicans could be able to overcome industry resistance if they did so together. But one issue is that the White House has shied away from clearly advocating the elimination of many big breaks. "My preference is to maintain interest deductibility," Treasury Secretary Steven Mnuchin said in congressional testimony on Wednesday, hedging that it was on the table.

As for the border adjustment tax, which would raise about $1 trillion, a number of senators have expressed doubt or outright opposition, and no senator has championed it.

Nor have any senators proposed curbing itemized deductions the way House Republicans have. Those include deductions for medical expenses, state and local taxes and much more.

The Trump administration has placed itself squarely behind ending the state and local tax deduction, a major itemized deduction that is worth well over $1 trillion over a decade. Although less ambitious than the House plan to end all itemized deductions, it is an ambitious proposal.

Yet no senator has committed to it. Asked Thursday if attacking the break is feasible in the Senate, Hatch simply waved the question off.

If Republicans hope to pass meaningful tax reform, though, they will have to get Senate support for limiting some big tax breaks.

Each break makes an enormous difference in terms of the tax rates that can be achieved. In the House blueprint, for instance, the inclusion of the border adjustment tax allows them to lower the corporate tax rate to 20 percent. Without it, they would be able to lower the rate to only 30 percent without adding to the deficit, said Kyle Pomerleau, the head of the tax modeling team at the Tax Foundation. A 30 percent corporate tax rate would still be well above the average for advanced economies and even higher than former President Barack Obama's target of 28 percent.

Another example: Eliminating the state and local tax deduction would raise about $1.8 trillion over 10 years in the Tax Foundation's model, which is based on one of the models used by Congress' own tax experts. That revenue pays for nearly all of the individual tax cuts in the House Republican plan, Pomerleau said, including the reduction in the top tax rate from 39.6 percent to 33 percent, the collapse of seven brackets into three and cuts to capital gains taxes. "Without that," Pomerleau said of eliminating the state and local tax deduction, "it's really hard to get anything on the individual side."

At the same time, curtailing the ability of residents to deduct their state and local taxes would be a declaration of war on high-tax blue states and on the real estate industry, which hates the idea of families losing a break that helps them with property taxes.

The only way to get the public to go along with a tax reform that got rid of that tax break would be to simultaneously promise them that their rates will be lowered dramatically.

So far, unlike the House and White House, the Senate has not proposed a framework for reducing tax rates. That is part of the reason senators have been reluctant to run afoul of special interests by putting their tax breaks on the table, suggested Dean Zerbe, Alliantgroup national managing director and a former staffer on the Finance Committee.

"You always want to kind of have dessert at the same time you're serving the vegetables," he said.