House Speaker Paul D. Ryan (R-Wis.) on Thursday uses charts and graphs to explain the GOP’s plan to repeal and replace the Affordable Care Act. (J. Scott Applewhite/Associated Press)

“WE’RE GOING to have a health care that is far less expensive and far better.” That was President Trump in January. And it is the only sort of policy analysis the public would be left with absent the Congressional Budget Office (CBO).

As House Republicans began work on their Obamacare replacement plan last week, they avoided addressing the likelihood that it would significantly increase the ranks of the uninsured and dodged questions about its fiscal responsibility by plowing ahead before Congress’s staff experts at the CBO had a chance to estimate the proposal’s effects. In the absence of sober-minded analysis, Republicans offered tricky rhetoric about expanding choice and freeing the market.

But they can avoid facing up to the negative consequences of their plan for only so long: The CBO will soon issue a report on their proposal, probably this week. So Republicans preemptively attacked the country’s designated budget scorekeepers. “If you’re looking to the CBO for accuracy, you’re looking in the wrong place,” White House spokesman Sean Spicer inveighed. House Majority Whip Steve Scalise (R-La.) sneered at the office’s “unelected bureaucrats.” Rep. Dave Brat (R-Va.) declared that “CBO has scored everything wrong forever, so they’re a minor concern.”

These attacks are beyond galling.

The CBO is run by an economist Republicans hand-selected after Tom Price — then the Budget Committee chairman, now Mr. Trump’s health and human services secretary — called for replacing the well-respected previous director. Their choice, Keith Hall, has overseen the move toward “dynamic scoring,” which makes Republican tax cuts seem less costly. But not toward rank reality-denial, which is what it would take to swallow some of the claims Republicans have made about repealing and replacing Obamacare.

The GOP hypocrisy is mind-blowing. Over the past two decades, CBO is perhaps best known for analyses that put a stamp of budgetary responsibility on Obamacare, but also papers that enabled Republicans to cut taxes during the George W. Bush years and, later, to slam President Obama on the long-term debt picture. Time after time, Republicans had nothing but praise for the office’s “nonpartisan” work. They were quick to cite the CBO when it concluded that some people would willingly work less under Obamacare.

Republicans’ latest volleys would hardly be the first partisan attacks the CBO has sustained. But, in the midst of Mr. Trump’s wide assault on institutions that arbitrate factual disputes and provide unbiased information, their condemnation is not just odious, but dangerous, too.

The CBO, of course, will not provide perfect predictions about the effects of complex policies in a big economy. But its projections about the new health-care legislation will be far more informed than others’ — Mr. Trump’s, for example.