A trio of Republican senators on Wednesday raised concerns over a proposal from their GOP colleagues to include in a pending overhaul of the U.S. tax code a provision to raise tax rates if certain economic benefits are not achieved.

But the possible inclusion of the so-called “trigger option” does not appear to be enough to lead either Sens. Thom Tillis of North Carolina, Dean Heller of Nevada or David Perdue of Georgia to vote no on the legislation.

A vote to proceed to the bill is expected on Wednesday. 

“I do share concerns with some of the suggestions for changes, we’ll work through them though,” Tillis said. 

Watch: Decoding Reconciliation — Why the Senate Only Needs 50 Votes on Tax Bill

He suggested that instead of including a trigger option, Republicans could make changes to the bill in future years through the same fast-track budget procedure known as reconciliation that the GOP is using to advance the legislation with only a simple majority support.

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One possible option lawmakers are said to be considering is a provision that would raise the corporate tax rate above 20 percent after five years if economic growth does not average 0.4 percent per year. 

Perdue, who before coming to Congress was chief executive officer of Dollar General, said such a measure would have impacted his ability to confidently make capital investment decisions. 

“I personally prefer not to have a trigger,” he told reporters. “You really don’t ever get 100 percent of what you want. And I don’t want to see this bill destroyed because of pursuit for perfection.”

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Optimism remains high that Senate Republicans will be able to eventually muster up the 50 votes needed to advance a bill, assuming that Vice President Mike Pence would cast the deciding vote. 

“It’s the most unified effort I’ve seen on any issue in many years from Republicans in the House and the Senate and the president,” Tim Phillips, president of the conservative group Americans for Prosperity, said at the presser. 

Some GOP members — like Sens. Ron Johnson of Wisconsin and Steve Daines of Montana — continue to have concerns over how the bill would impact pass-through businesses. 

Others, including Sen. Susan Collins of Maine, have raised concerns over the inclusion of a measure to repeal the individual mandate created by the 2010 health law. Those concerns are likely to be addressed through a separate bill from Senate Health, Education, Labor and Pensions Chairman Lamar Alexander and Sen. Patty Murray of Washington, the panel’s top Democrat.

Republican aides say the legislation that aims to stabilize the markets created by the health law won’t be included in the tax bill, but rather in an upcoming measure to address government funding in fiscal year 2018.