Sen. Jeff Flake said Thursday he's confident lawmakers will approve a budget deal seen as crucial to helping fellow Republicans pass tax reform.
The budget legislation, up for a vote in the Senate on Thursday, would permit Republicans to pass follow-up tax cuts later this year.
"I think on the budget deal it's in the bag," Flake said on CNBC's "Squawk Box." "But whether or not we can get to tax reform, we're going to see over the next couple of months."
President Donald Trump commented Thursday on the budget deal, calling "it the first step toward massive tax cuts."
"I think we have the votes, but who knows?" he wrote on Twitter.
The GOP unveiled its blueprint for tax reform last month. It calls for cutting personal and corporate tax rates and aims to simplify the U.S. tax code.
In order for the U.S. to be competitive, lawmakers must reduce the corporate tax rate, said Flake, a Trump critic who is facing a tough battle in Arizona's 2018 primary. The Republican tax bill should provide real reform and not just a tax cut, he added.
"Reform involves broadening the base. And that involves getting rid of some very popular credits and deductions and loopholes," Flake said. "When you add all the credits, deductions and loopholes ... it equals more than our entire discretionary budget."
"It's obviously a tough political sell," Flake said.
—The Associated Press contributed to this report.