EVERY FEW years, whichever party is running Congress encounters an obstacle: The nonpartisan scorekeepers at the Congressional Budget Office (CBO) and the Joint Committee on Taxation (JCT). These professionals issue the official estimates on what major legislation would accomplish and how much it would cost, and they have a way of puncturing the narratives federal lawmakers construct to justify their policies. They tell Republicans that tax cuts are not likely to pay for themselves, and they tell Democrats that expanding government-subsidized health-care coverage may reduce the number of hours Americans work.

When majority lawmakers get a score they dislike, they and their ideological allies often try to work the refs — making their case to the CBO and JCT staff, or issuing statements challenging the estimates. Sometimes, and increasingly, they go further, threatening to eviscerate the scorekeepers’ independence. Mother Jones’s Kara Voght reported last week that Senate Democratic staffers are talking about firing CBO Director Phillip Swagel, after the CBO released an analysis projecting that increasing the federal minimum wage to $15 an hour would result in 1.4 million job losses over a decade. Staff on the Senate Budget Committee, which Sen. Bernie Sanders (I-Vt.) runs, have even sent Senate Democratic leaders a list of possible replacements. This is a terrible idea.

Firing Mr. Swagel would undermine one of the few official institutions left in Washington that have remained steadfastly, admirably apolitical in the face of intense partisan pressure. No, the CBO does not jump to embrace every trend among the most progressive economics faculties or the most conservative think tanks, which tends to infuriate ideologues seeking have-their-cake-and-eat-it-too economics.

The CBO also does not get everything right. Its experts provide highly educated guesses. Sometimes this helps Democrats, as when the CBO concluded that repealing Obamacare would increase the deficit. Sometimes it hurts their case, as it did on the minimum wage. CBO estimates force caution on those who have an interest in the numbers turning out a specific way, and its careful approach means that both parties’ legislation is measured against the same yardstick over time. It cannot perform either function if each passing congressional majority stacks the CBO in its favor.

Republicans threatened the scorekeepers’ independence in 2015, when they commanded the JCT, which scores tax bills, and CBO, which scores other legislation, to use “dynamic” analysis when assessing major proposals. (Dynamic scoring considers how the economic consequences of a policy might affect the federal budget — such as a tax cut boosting economic growth and therefore revenue, offsetting the cost of the cut.) Fortunately, JCT and CBO leaders committed to maintaining the office’s credibility took a characteristically cautious approach. JCT rejected arguments that the GOP’s 2017 tax bill would pay for itself.

Now the Democrats want to push through historic taxing-and-spending legislation, perhaps using reconciliation, a parliamentary maneuver that allows budget-related bills to pass with a simple majority in the Senate. The CBO’s analysis may affect how the effort plays with moderate lawmakers and determine which provisions are eligible for reconciliation. At this point, the discussions on firing Mr. Swagel may be nothing more than a shot across the director’s bow rather than a serious effort to stack the CBO. Thankfully, key House Democrats, who would be in charge of replacing the director, insist they are uninterested.

Senate Democrats should put firing Mr. Swagel out of their minds, and the CBO should ignore the pressure.

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