With a vote on the controversial Republican tax bill expected in the Senate later this week, GOP leaders are furiously whipping the handful of lawmakers who could make or break the bill’s success. But for every vote they pull on board, more seem to fall off the wagon.

On Monday, yet another Republican senator aired concerns about the bill, particularly estimates that it would balloon the federal deficit by $1.2 to $1.4 trillion. Sen. James Lankford (R-OK) told reporters in a Capitol Hill press conference that he’s skeptical the promised economic growth will fill that hole, and refused to say how he will vote on the tax bill itself.

“What if we don’t get 0.4 percent growth? Is there any backstop?” he asked. “I’m not opposed to tax reform, but we need to do it right.”

Lankford also suggested that conservatives who have long railed about the size of the federal deficit are acting hypocritically by backing the deep tax cuts without corresponding spending cuts.

“We can’t ignore the debt and deficit issues,” he said. “As conservatives, we’ve said for a long time that to get ahead of the deficit we have to control our spending and have a growing, healthy economy. Well, if we use all of the tax reductions to just offset, we’ll never get on top of it.”

Lankford, a member of the key Senate Finance Committee that hashes out tax policy, said Republicans need to learn from what happened over the past few years in Kansas, where deep tax cuts sent the state into a fiscal crisis, forcing cuts to public school funding, roads, retiree pensions, and state universities.

“Those of us in Oklahoma and Kansas and the middle of the country have seen some of this in our own state legislatures,” Lankford said. “It’s important to learn from what we’ve seen.”

Kansas Republican Sen. Jerry Moran shared similar fears in town hall meetings and local interviews over the Thanksgiving break.

“We don’t want to increase the debt and deficit as a result of tax cuts,” he told a crowd in Clay Center on Saturday. “I’m also cognizant of what people saw happen in Kansas.”

Republicans can only lose two votes from their caucus and still pass the tax bill. With deeply conservative lawmakers like Moran and Lankford wobbling, in addition to more moderate members like Sens. Susan Collins (R-ME) and Jeff Flake (R-AZ), the upcoming Senate vote is likely to be a nail-biter for GOP leaders desperate to avoid closing out the year with no major legislative accomplishments.