Lankford said on Monday he wants the legislation to include some sort of “backstop” that would trigger an increase in tax rates if federal revenues fall short of projections in the next few years. “To me, the big issue is, how are we dealing with the debt and deficit? Do we have realistic numbers, and is there a backstop in the process just in case we don’t?” he told reporters at the Capitol. Republicans are relying on projections that their proposed cuts to the corporate and individual income-tax rates will boost economic growth by 0.4 percent a year. They believe that would offset the $1.5 trillion the bill would otherwise add to the debt.

But other fiscal analysts have disputed that forecast. The hawkish Committee for a Responsible Federal Budget predicts the GOP tax proposals will generate no more than 0.1 percent in annual economic growth—a difference that would amount to hundreds of billions in lost federal revenue. And in Oklahoma and Kansas, similarly rosy predictions never panned out, whether because the tax cuts were not as effective as Republican legislators believed they would be, or because a drop in oil-and-gas prices hammered the states’ economies.

“I think it’s important that we learn some of those lessons that we’ve seen in states and to be able to put into place, at the beginning, a backstop procedure to make sure that we’re guarding against this,” Lankford said.

The Kansas legislature, aided by a coalition of newly-elected moderate Republicans and Democrats, earlier this year actually rolled back the steep tax cuts enacted in 2011 and 2012 under conservative Governor Sam Brownback. Some of those GOP legislators have urged their counterparts in Congress to heed their example. The state’s House delegation ignored those warnings in voting to pass the GOP tax plan earlier this month. But Moran suggested he’d listened to their concerns during a town hall he held in Kansas over the weekend. “I’m also cognizant of what people saw happen in Kansas,” Moran said, according to the Topeka Capital-Journal. “The issue of tax cuts would be easier if you actually had faith that Congress would hold the line on spending. It’s two components. It’s how much revenue you take in and how much money you continue to spend.” While Republicans ultimately want to cut the size of government, Congress is likelier to increase spending during a year-end appropriations deal to accommodate President Trump’s demand for more money for the military.

Lankford did not specify his requested changes, and a spokesman for the senator said he was “not ready to talk about his proposal in detail.” The Senate bill already includes revenue triggers for certain, smaller provisions. But setting up additional possible tax increases would be complicated for a number of reasons, said Scott Greenberg, a senior analyst with the Tax Foundation. For one, if revenues fall off in the next few years, it could be a sign that the economy is in recession, and tax increases would likely slow the economy even further. And a trigger could be “self-fulfilling,” Senator Pat Toomey of Pennsylvania told Bloomberg: The mere possibility that taxes could go up in a few years could cause businesses not to make investments they would make if the new rates were sure to stay in place; that’s a major reason why Senate Republicans set their individual tax cuts to expire and made the corporate-rate cuts permanent.