A previous version of this article mistakenly said the CBO projected the Republicans' 2017 tax bill would add $1.4 billion to the deficit over the next decade. In fact, the CBO said it would increase the national deficit by $1.4 trillion. This version has been corrected.

As Democrats move quickly to pass President Biden’s agenda, one little-known office could have a big impact on their social safety net legislation: the Congressional Budget Office.

Biden says the cost of Democrats’ expanded government safety net legislation is actually zero, contending that most of it will be paid for. But as the House of Representatives passed its version Friday, the CBO disagreed with the president, saying it will add some to the national debt. And that could give moderate Senate Democrats pause in supporting the legislation.

Here’s what the CBO score is and how it could hold up Democrats’ agenda.

What the CBO does

The Congressional Budget Office is a nonpartisan agency that estimates how much proposed and existing legislation will cost American taxpayers. Congress created the office in a law in 1974 as part of a broader effort to assert more control over the power of the purse, which is Congress’s constitutionally mandated duty.

The office is strictly nonpartisan. It doesn’t recommend how to better spend money. It doesn’t take sides in a policy debate. It doesn’t even make policy recommendations. Its only job is to estimate the economic impact of legislation. It does this in a couple of ways, primarily:

  • Scoring proposed legislation to estimate how much it will cost over a period of time. Lawmakers request this a lot, apparently. The CBO estimates that it provides thousands of informal estimates a year to various lawmakers and congressional committees. Congress also requires a CBO score for nearly every bill approved by a committee before it comes to the floor for a vote.
  • The CBO also issues annual reports estimating the economic impact of all current laws over the next decade.

The CBO is the only official authority in Congress on budget matters. So while lawmakers can grumble that they don’t agree with a particular CBO estimate on their legislation — and they can and often do hold up interest groups’ own economic analyses — the only one that carries any weight in Congress is the CBO’s score.

“Somebody has to be a nonpartisan, intellectually credible umpire of budget scoring,” Robert Bixby of the bipartisan budget watchdog Concord Coalition told U.S. News & World Report in 2017. “And CBO is it. In that capacity, they’re extremely important to the process.”

Many of its official estimates are published publicly.

How the CBO could hold up the rest of Democrats’ legislation

A number of Democrat centrist lawmakers are wary of the cost — and of Republican attacks about big government spending — and they want a CBO score that they can show their constituents to explain why they voted for the package.

They were hoping the CBO showed that Democrats’ revenue measures, like taxing millionaires, would keep the impact on the national debt at little to nothing.

The CBO didn’t quite say that. They estimated that the bill will add $160 billion to the deficit over the next 10 years. But the Biden administration and Democratic leaders argued the bill doesn’t take into account the IRS’s ability to crack down on tax cheats. Treasury Secretary Janet Yellen argued that this could actually turn the legislation into a revenue-maker. (The CBO isn’t so bullish on the IRS’s ability to catch tax cheats.)

That was enough to get centrist House lawmakers to support the legislation, but not without expressing concerns. Rep. Stephanie Murphy (D-Fla.) voted for it but said she still has “reservations about the overall size of the legislation.”

In the Senate, which has yet to vote on its own version of this legislation, Sen. Joe Manchin III (D-W. Va.) has expressed his own concerns about the cost.

CBO scores have derailed legislation before

In 2017, Republicans had control of Washington and were trying to repeal the Affordable Care Act. The CBO (which also estimates the impact on health care for appropriate laws) reported that the Republicans’ plan would leave 22 million additional Americans uninsured over the next decade. Headlines like that helped increase the popularity of the ACA, and Republicans eventually failed to overturn the law.

Then, as Republicans pushed forward with a tax-cut bill, the CBO estimated that it would increase the national deficit by $1.4 trillion over the next decade. This bill did get passed into law, and that estimate proved to be prescient. The national debt grew exponentially during Donald Trump’s presidency, approaching World War II levels at 128 percent of gross domestic product. It was driven by the tax-cut bill.

This article has been updated.