Republican Sen. Ron Johnson will oppose the Senate tax plan as written, his office confirmed Wednesday.
The lawmaker from Wisconsin is the first GOP senator to explicitly say he will not back the tax proposal. Johnson told The Wall Street Journal that the bill benefits corporations more than other businesses.
"If they can pass it without me, let them," he told the newspaper, which first reported his opposition. "I'm not going to vote for this tax package."
Johnson's opposition adds uncertainty to the GOP goal of chopping tax rates on businesses and individuals by the end of the year. Other Republican senators have raised concerns about budget deficits fueled by tax cuts, though none other than Johnson have publicly opposed the bill, yet.
Johnson could end up voting for the Senate bill, as it still may change before the full Senate votes. He initially opposed one version of the Senate's push to repeal the Affordable Care Act this summer, before eventually voting for it.
Republicans hold 52 of 100 seats in the Senate. If all Democrats and independents oppose it, the GOP can only lose two votes and still pass the plan with the simple majority needed under special budget rules.
GOP senators hope to pass their bill as soon as the week after Thanksgiving.
Senate Finance Committee Chairman Orrin Hatch, R-Utah, released a revised version of the bill Tuesday night. Among the biggest changes, it would make most individual rate cuts expire after 2025, while the corporate tax rate would get reduced permanently.
It also would effectively repeal the Obamacare individual mandate. The move gives senators more room to comply with budget rules, but it could lead to an estimated 13 million more people uninsured and increase average health-care premiums, according to a Congressional Budget Office estimate.
In the Journal interview, Johnson expressed concerns about the treatment of pass-through businesses, which pay individual income tax rates. The Senate bill would give those businesses tax breaks, while the House plan cuts the rate they pay to 25 percent.
In a statement released later Wednesday, Johnson said that "neither the House nor the Senate bill provide fair treatment" for pass-through businesses, "so I do not support either in their current versions."
"I do, however, look forward to working with my colleagues to address the disparity so I can support the final version," he said.
Johnson also told the newspaper that he has felt shut out of the process of making the bill, calling it "offensive."
Programming note: Johnson will appear on CNBC's "Squawk Box" at 7:30 a.m. ET on Thursday.