Correction: This op-ed has been updated to correct the name of the Republican Senate leader who supported the Congressional Budget Office during the Reagan administration. The leader was Howard Baker of Tennessee.

Alice M. Rivlin was the founding director of the Congressional Budget Office and director of the White House Office of Management and Budget in the Clinton administration. She co-chairs the Bipartisan Policy Center’s Debt Reduction Task Force and serves as a senior fellow at the Brookings Institution.

Congress faces a daunting legislative agenda: Health care is “complicated,” as President Trump recently discovered. So are tax reform and spending bills that reflect campaign promises without escalating debt.

There are deep divisions over many of the Trump administration’s proposals — including within the president’s own party — and there will be serious disputes about the costs of many proposals and their effects. Congress needs all the help it can get from the team of neutral experts it created for just these moments: the Congressional Budget Office.

Some members of Congress and the administration have recently questioned the need for CBO estimates, such as the assessment of the Republican health-care bill released Monday. White House press secretary Sean Spicer said last week that he would sooner accept an analysis from the Office of Management and Budget, whose director was appointed by Trump, than the CBO.

But these two offices have two very different jobs, as I discovered as both the first director of the CBO and later as the director of the OMB in the Clinton administration. The CBO serves Congress; the OMB serves the president. Given the constitutional separation of executive and legislative powers, both offices are needed.

Spicer’s view is not surprising. All presidents passionately favor their own proposals and are optimistic about what they will cost and how much good they will do. When I ran the OMB, we often thought our congressional counterpart did not appreciate how great our proposals were — especially the ill-fated Clinton health plan, which the CBO thought would have a much more negative impact on the budget than we alleged.

But Congress is an independent branch of government that needs its own team of neutral experts before it decides on legislation. Fortunately, the CBO has a strong team. It won’t get everything right — it originally overestimated the costs of the Affordable Care Act, primarily because it expected more people to sign up — but it will do its best to sort through the complexity of these proposals and give Congress the benefit of its best judgment.

For 42 years, the CBO has earned congressional and public respect by providing neutral, nonpartisan estimates of costs and effects of pending legislation, including analyses of alternative policy options. It has a long-established tradition of able leadership and highly qualified, hard-working staff. It is a valuable resource for Congress that both parties need.

Congress created the CBO during the constitutional crises that emerged when President Richard Nixon refused to spend funds that had been appropriated by Congress and signed by him. Outrage at executive overreach prompted Congress to pass the Congressional Budget and Impoundment Act of 1974, which reestablished Congress as an equal partner with the executive branch in deciding the country’s fiscal and legislative policies. The new law strengthened the congressional budget process by creating two budget committees and a nonpartisan, independent Congressional Budget Office to provide estimates and budget analyses to both the House and the Senate — for Democrats, Republicans and independents alike.

This administration is not the first to misunderstand the role of the CBO. The supply-side economists in the nascent Reagan administration held extremely optimistic views about the big jump in economic growth anticipated by President Ronald Reagan’s proposed tax cuts. Because administration officials feared that the CBO might question their “rosy scenario,” they suggested ousting me as CBO director. But Senate Budget Committee Chairman Pete Domenici (R-N.M.), supported by Republican leader Howard Baker (Tenn.), quickly informed the White House that the CBO worked for Congress — not the president — and that meddling with it was not appropriate.

Undermining the credibility of the CBO when policymakers need it most harms not only Congress’s ability to do its job, but also the long-term effectiveness of political parties in addressing the challenges that face our country’s future.

Republicans in Congress must make the transition from opposing to governing, as House Speaker Paul D. Ryan (R-Wis.) has emphasized. Governing is hard, and it will be much harder if every member of Congress can pick and choose among estimates of the costs and consequences of the complex proposals under consideration. To address the daunting legislative agenda before them and legislate in the best interest of the country, Congress needs to use the great resource that it has in the CBO.