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WASHINGTON — Republican Sen. Marco Rubio declared Thursday he will vote against the GOP’S sweeping tax package unless negotiators expand its child tax credit, jeopardizing the Republicans’ razor-thin margin as they try to muscle the $1.5 trillion bill through Congress next week.

Rubio wants to increase the portion of the basic $2,000-per-child tax credit that would go to low-income families. In the current bill, families that pay little or no income tax would get only $1,100 per child. Rubio wants that amount increased, but he didn’t specify an amount.

Under current law, the tax credit is $1,000.

“Given all the other changes they made in the tax code leading into it, I can’t in good conscience support it unless we are able to increase the refundable portion of it. And there’s ways to do it, and we’ll be very reasonable about it,” Rubio said.

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He said he has received no assurances the credit for low-income parents will be increased.

Rubio’s defection would leave the Republicans with little wiggle room to pass their first major legislation of President Donald Trump’s presidency unless they are able to satisfy the Florida senator. The Senate passed its earlier version of the tax package by only 51-49, with Rubio voting for it.

The legislation, still being finalized, would cut the top tax rate for the wealthiest earners — Trump among them — from 39.6 percent to 37 percent, slash the corporate income tax rate from 35 percent to 21 percent and allow homeowners to deduct interest only on the first $750,000 of a new mortgage.

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If Senate Republicans lose Rubio and also Sen. Bob Corker of Tennessee, who voted against the original bill, the vote would stand at 50-50, with Vice President Mike Pence prepared to cast the tie-breaker for passage. Corker said Thursday he is undecided.

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Complicating the GOP effort further, 81-year-old John McCain of Arizona is at a local military hospital being treated for the side effects of brain cancer treatment, and 80-year-old Thad Cochran of Mississippi had a non-melanoma lesion removed from his nose earlier this week. Both Republicans missed all Senate votes this week, but GOP leaders are hopeful they will be available next week.

Rubio’s announcement came a day after House and Senate Republican leaders forged an agreement in principle on the most sweeping overhaul of the nation’s tax laws in more than 30 years. The package would give generous tax cuts to corporations and the wealthiest Americans, and more modest tax cuts to low- and middle-income families.

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The agreement also calls for scrapping a major tax requirement of the “Obamacare” health law, a step toward the ultimate GOP goal of unraveling the law.

The tax package is polling badly among the public, but Republicans say that will turn around after Americans see the benefits.

“What comforts me greatly is the fact that the results are going to produce,” said House Speaker Paul Ryan of Wisconsin. “I’m convinced this is going to help repatriate capital. I’m convinced this is going to launch more investment in businesses and workers. I’m convinced this is going to give bigger paychecks, a simpler system.”

“The results are going to be what sells this bill, not the confusion before it passes,” Ryan said.

In the current version, the standard deduction would be nearly doubled, to $24,000 for married couples.

The tax bill would scale back the deduction for state and local taxes, allowing families to deduct only up to a total of $10,000 in property and income taxes. The deduction is especially important to residents of high-tax states such as New York, New Jersey and California.

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Business owners who report business income on their personal tax returns would be able to deduct 20 percent of that income.

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The business tax cuts would be permanent, but reductions for individuals would expire in 2026 — saving money to comply with Senate budget rules. In all, the bill would cut taxes by about $1.5 trillion over the next 10 years, adding billions to the nation’s mounting debt.

Details of the agreement were described by Republican senators and congressional aides. The aides spoke on condition of anonymity because they weren’t authorized to publicly discuss private negotiations.

Across the Capitol Thursday, House Republicans picked up support for the tax bill from conservatives in the chamber’s Freedom Caucus.

“I think it’s going to pass. I think you’re going to see the vast majority of the Freedom Caucus people vote for it,” said Rep. Jim Jordan of Ohio.

The leader of the caucus, Rep. Mark Meadows of North Carolina, said that “at this point I don’t have any alarm bells going off.”

The Freedom Caucus is a key constituency for House GOP leaders because a united group with more than 30 members could kill any bill that lacks support from Democrats. Congressional Democrats, who were excluded from crafting the tax package, have been united against it.

Republicans view passage of the legislation as a political imperative, proving to voters they can govern as the GOP fights to hold onto its majorities in the House and Senate in next year’s elections.

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Associated Press writers Kevin Freking and Ken Thomas contributed to this report.