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.

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.

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.

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.