Senate Republicans must grapple with another defection now that Ron Johnson, a senator from Wisconsin, became the first senator in his party to declare that he could not vote for the GOP's tax reform plan, arguing that the bill is imbalanced in favor of large corporations.

"Unfortunately, neither the House nor Senate bill provide fair treatment, so I do not support either in their current versions," he said. "I do, however, look forward to working with my colleagues to address the disparity so I can support the final version."

Johnson also told The Wall Street Journal that he cannot vote on the bill as it's currently written. "If they can pass it without me, let them," Johnson told WSJ reporters. "I'm not going to vote for this tax package."

He also detailed the changes he would make, which are provided below.

In a statement, Johnson stressed his support for small businesses, whose owners pay taxes on profits through the tax code for individuals. These businesses truly are the engines of innovation and job creation throughout our economy, and they should not be left behind,” he said in a statement. “Unfortunately, neither the House nor Senate bill provide fair treatment, so I do not support either in their current versions.”

Johnson's opposition places Republican leaders in a bind: Although many Senate Republicans have expressed their enthusiasm for a major legislative achievement ever since the public failure to repeal the Affordable Care Act threatened to further divide the party, the party’s slim 52-to-48 majority in the Senate makes members especially vulnerable to the effects of defections, which could doom the measure altogether.

The support of several other Republican senators is also uncertain––and the three senators who could determine the bill's future are the same ones who killed efforts to repeal the ACA: Senators Susan Collins (ME), John McCain (AZ) and Lisa Murkowski (AK).

Collins told reporters in the Capitol "that her staff's research showed pairing tax cuts with an effective repeal of the individual mandate of Obamacare, formally known as the Affordable Care Act (ACA), could be a mistake," according to CNBC.

"I have data that demonstrates for certain middle-income individuals and couples, who do not qualify for subsidies under the ACA ... that the premium increase will outweigh the tax cut that they get," she said. "I suspected this, based on what I know about insurance markets, but now I have the actual data."

McCain himself has declined to say whether he would vote for a tax bill that includes the proposed change to the ACA. “I want to see the whole package before I make a decision,” McCain said.

Murkowski expressed her concerns. "Are you going to have a situation where your premiums now increase?" she said. "Tell me how that's making me a happier person in the middle class here. That's a consideration I think is very real and needs to be weighed."