Two Republican senators, Ron Johnson Ronald (Ron) Harold JohnsonThis week: Supreme Court fight over Ginsburg's seat upends Congress's agenda GOP set to release controversial Biden report Democrats fear Russia interference could spoil bid to retake Senate MORE (Wis.) and Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (Tenn.), are threatening to vote against the GOP tax reform bill in the Senate Budget Committee on Tuesday unless changes are made to address their concerns.

Corker's and Johnson's primary issues with the legislation are different: Corker fears predicted deficit increases if the measure is passed, while Johnson worries the bill does not do enough to help small businesses.

Another Republican, Sen. Steve Daines Steven (Steve) David DainesMcConnell locks down key GOP votes in Supreme Court fight Will Republicans' rank hypocrisy hinder their rush to replace Ginsburg? Toobin: McConnell engaging in 'greatest act of hypocrisy in American political history' with Ginsburg replacement vote MORE (Mont.), who is not on the budget panel, has also said he cannot support the legislation in its current form. Like Johnson, Daines fears the bill will leave "pass-through businesses," whose income is taxed through the individual tax code, with a higher rate than other firms.

Republicans have a narrow 12-11 majority on the Budget Committee, so they can't afford any defections from the panel's GOP members.

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“I’m not exactly sure what’s going to happen in committee, we’re working diligently to fix the problem. If we develop a fix prior to committee, I’ll probably support it, but if we don’t, I’ll vote against it,” Johnson told Wisconsin reporters Monday afternoon.

Corker, when asked whether he may vote against the bill in committee, also told reporters that it's "very possible." He and several other deficit hawks want the bill to include a "backstop" in the event that economic growth projections aren't met.

"That’s been known for sometime that having a backstop that we know is real is an important step for us along the way," he added.

Republicans can afford two defections at most on the full Senate floor, assuming every Democrat and independent votes against it.

Corker, one of President Trump's most vocal GOP critics, has announced he is not running for reelection next year. Neither Johnson nor Daines face reelection in 2018.