From left, Rep. Cathy McMorris Rodgers, R-Wash., Speaker Paul Ryan, R-Wis., Rep. Neal Dunn, R-Fla., House Majority Leader Kevin McCarthy, R-Calif., and Majority Whip Steve Scalise, R-La., conduct a news conference after a meeting of the House Republican Conference in the Capitol on December 19, 2017.

House Speaker Paul Ryan on Wednesday said "nobody knows" whether the Republican tax cuts will spur enough growth to pay for themselves "because that's in the future."

Ryan's comment came just hours before House Republicans were scheduled to hold a final vote to pass a bill containing massive, permanent corporate tax cuts, and temporary tax reductions for individuals.

Appearing on NBC's "TODAY" show, Ryan, R-Wis., was asked if the tax cuts would add to the deficit at all because of the growth the GOP expects it to generate.

"Nobody knows the answer to that question because that's in the future," he replied, "but what we do know is that this will increase economic growth."

This appears at odds with months of messaging from Republican leaders and the White House, however, who have argued that the tax cuts will act as "rocket fuel" for the economy, producing so much growth that the federal government will recoup any initial revenue losses.

"Not only will this tax plan pay for itself, but it will pay down debt," Treasury Secretary Steven Mnuchin said earlier this year, as the tax plan was being negotiated.

The Treasury Department even released a summary this month of exactly how the tax cuts would pay for themselves — a one page document that was quickly panned by economists.

And while Ryan has largely deflected specific questions about economic growth figures, he has challenged the accuracy of some models that show the tax cuts would add more than a trillion dollars to the national debt over a decade.

In recent weeks, the tax cuts have come under even more fire for making corporate tax cuts permanent, but individual tax cuts only temporary. To help counter backlash over this, Ryan adopted language that emphasizes the universal benefits of the tax cut bill.

"It's about faster growth and upward mobility. It's about a strong economy that makes all of us stronger and healthier." Ryan said Tuesday on the House floor. "Those are the effects, those are the benefits of tax reform."

— CNBC's Jacob Pramuk contributed reporting.

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