Washington (CNN) Sen. Ron Johnson, R-Wisconsin, explained his opposition Thursday to his party's tax bill, telling CNN's "New Day" that the plan as currently written is unfair to businesses.

"I'm just looking for a fair shake for all businesses to maintain the competitive balance and position of all businesses," Johnson said, later adding: "I'm not for the current version."

Johnson announced his opposition to the bill on Wednesday, making him the first member of the GOP to formally come out against the party's plan, but he's left open the door to supporting the bills if they are altered.

"(W)e're leaving those pass-through businesses behind. And those pass-through businesses are the engine of economic growth, job creation, innovation in our economy," he said.

Most US businesses, large and small, are set up as pass-throughs, not corporations. They run the gamut from mom-and-pop shops to big law firms and investment partnerships. They're called pass-throughs because the profits are passed through to the owners, shareholders and partners, who pay tax on them through their personal returns.

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