In the final stages of crafting the Senate bill, Rubio pitched a plan to slightly shrink the size of the measure's proposed corporate tax cut and use that revenue to bump benefits for about 9 million low-income American families through the Child Tax Credit. But the proposal failed amid opposition from top GOP leaders, who argued the proposed change to the corporate tax rate would hurt U.S. companies' ability to compete.

As party leaders work to reconcile the House and Senate tax bills, however, they appear to have agreed on a change to the corporate rate very similar to the one Rubio proposed. But instead of using that money for tax credits for the poor, the new plan would further cut income taxes for millionaires.

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The deal is not final yet, and two Republican Senators expressed optimism on Wednesday that a more generous tax credit could still wind up in the final package.

“I’m hopeful we are able to see a richer approach to the fundable portion of it. I think it’s still a possibility,” said Sen. Tim Scott (R-S.C.), in remarks later echoed by Sen. Mike Lee (R-Utah) in a Facebook live video with constituents.

But if the GOP decides not to bolster the child tax credit, Rubio has to decide whether to continue supporting his party's top legislative priority, even after party leaders ignored one of his major proposals.

Rubio and his office declined repeated requests for an interview on where he stands on the bill.

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He protested when his proposal was rejected the first time around and fumed again Tuesday, noting that his proposal was rejected over suggesting the bill's corporate tax rate change from 20 percent to 20.94 percent, while the new proposal would raise it from 20 percent to 21 percent. (Under current law, the corporate tax rate is 35 percent.)

Some of his allies are already urging him to reject the reported deal between House and Senate negotiators.

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“Rubio got as close to a middle-finger from the conference committee as it gets. He has so far shied away from using his vote as leverage and has been repeatedly burned as a result,” said Samuel Hammond, a poverty analyst at the Niskanen Center, a libertarian-leaning think tank, who spearheaded a conservative lobbying campaign with Rubio to expand the credit. “This should change his calculus.”

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The importance of Rubio's support is magnified by Republicans' slim margin in the Senate. They hold 52 seats and need 50 votes to pass their tax bill, but with Sen. Bob Corker (R-Tenn.) already opposing the measure, the party can only afford to lose support from one more Republican. (Vice President Pence could break a 50-50 time.)

Democrat Doug Jones's upset victory in the Alabama U.S. Senate race on Tuesday night gives the negotiations greater urgency, as Senate Majority Leader Mitch McConnell (R-Ky.) is expected to rush a tax bill to a vote before Jones can replace Sen. Luther Strange (R-Ala.), a reliable vote for the bill. That means the GOP has little time to go back and make major changes to the plan if the working version loses too much support.

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Senators from across the Republican Party's ideological spectrum have been able to extract concessions from GOP leadership in exchange for their vote for the bill. Sen. Ron Johnson (R-Wis.) demanded lower tax rates for pass-through companies, and the rates dropped. Sen. Susan Collins (R-Maine) wanted Republican leaders to promise to support bipartisan bills to stabilize the Affordable Care Act, and she says they agreed to do so.

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But in opposing Rubio's amendment to the Senate bill, which he offered alongside Sen. Mike Lee (R-Utah), party leaders appear to have concluded that the onetime presidential hopeful wouldn't vote down the proposal, a prediction Rubio proved correct just hours later when he voted to pass the measure.

“I have no idea why he didn’t stick to his guns. I’m sort of at a loss to explain it,” said Joshua McCabe, a tax expert at Endicott College who has written about the history of the Child Tax Credit.

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Republicans appear ready to make that calculation again.

Rubio hasn’t publicly vowed to vote against the bill if the tax credit isn't increased, and both Rubio admirers and critics are skeptical he'd do it.

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“This isn’t a guy who wants to be off the reservation. [Rubio] wants to be a team player, and wants to be supportive of tax relief in general,” said April Ponnuru, a senior adviser at the Conservative Reform Network and a Jeb Bush adviser who has worked closely with Rubio's office on trying to expand the credit.

Rubio’s defenders also note that the Child Tax Credit is still expanded under the Senate bill from its place in existing law. Currently, the tax credit offers $1,000 to each family that qualifies; the Senate bill would double it to $2,000 per family. Lee, the other key advocate of the tax credit, is expected to vote for the bill unless there are major unexpected changes.

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Rubio's critics say that he isn't getting traction because his vote isn't in the balance, and that he's concerned about publicly pushing to help the poor but unwilling to suffer potential intraparty repercussions by using his maximum point of leverage on their behalf.

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Another theory is that Rubio himself bears blame for not being willing enough to publicly challenge his own party. “This is classic Marco Rubio: He wants to be seen as a leader on an issue, but he doesn’t have the backbone to draw a line in the sand,” said Christian Ulvert, a Democratic operative in Florida who began tracking Rubio's career in 2003.