In the end, the bill's success may hinge on the math.
Of course, the CBO isn't the only organization crunching the numbers.
Ratings agency Standard & Poor's last week released a report predicting between 6 million and 10 million people would lose coverage under the GOP's plan. Some liberal think tanks, like the Brookings Institute, put that number even higher.
Democrats, confident that the CBO's forecast will paint a similar picture, have blasted Republicans for marking up the bill without a CBO score.
But the Trump administration has been preemptively undermining the CBO's credibility, pointing to flaws in the office's early assessments of the Affordable Care Act.
The CBO has been "woefully underperforming when it comes to evaluating health systems," Health and Human Services Sec. Tom Price said on Fox News Friday.
While Spicer -- and the president, for that matter -- have cited CBO statistics to bolster their agenda in the past, their current skepticism is not without some basis: Some of the office's initial predictions about Obamacare were off the mark.
In the same projection, the CBO put the number of uninsured at around 21 million by 2016. But according to preliminary data from the CDC, more than 28 million people remained uninsured in the first three quarters of 2016.
On Sunday, Ryan acknowledged that the CBO's analysis of the Republican healthcare bill would likely include the projection that many Americans would lose coverage.
"CBO will say, well, gosh, not as many people will get coverage," Ryan said on CBS's Face the Nation. "You know why? Because this isn't a government mandate. This is not, the government makes you buy what we say you should buy."
"There's no way you can compete with, on paper, a government mandate with coverage," the Speaker added. "What we are trying to achieve here is bringing down the cost of care, bringing down the cost of insurance, not through government mandates and monopolies, but by having more choice and competition."
"I can't answer that question," Ryan said. "It's up to people."