Ron Johnson and Steve Daines will vote for Senate tax bill

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Sen. Steve Daines, R-Mont., speaks during a news conference in the Capitol where GOP senators said families and small businesses would benefit from tax reform on November 7, 2017.

Pass-through businesses get taxed at individual rates. While those entities include small businesses, most pass-through income goes to the top 1 percent of American earners, according to research cited by The New York Times.

The GOP has already secured support from two other key votes: Sens. John McCain, R-Ariz., and Lisa Murkowski, R-Alaska.

Republican leaders are also scrambling to win over Sens. Bob Corker, R-Tenn., Jeff Flake, R-Ariz., and James Lankford, R-Okla., who are concerned about budget deficits generated by tax cuts. A mechanism designed to win their votes was ruled unworkable under Senate rules on Thursday.

The nonpartisan Joint Committee on Taxation estimated Thursday that the plan would fall $1 trillion short of paying for itself, even after economic growth is taken into account. While GOP Senate leaders like Sen. John Cornyn of Texas and John Thune of South Dakota downplayed the findings, Corker pushed for a way to make up for the budget hole.

Sen. Susan Collins, R-Maine, has also not yet committed to backing the bill.

Republicans, who hold 52 seats, can only lose two votes and pass the bill if all Democrats and independents oppose it. They only need a simple majority, including a tiebreaking vote from Vice President Mike Pence, under special budget rules.

— CNBC’s Ylan Mui contributed to this report