Top Republicans have waged war on the CBO. Now former CBO directors are fighting back.

The Congressional Budget Office — an independent body tasked with projecting the effects of legislative proposals — has become a Republican punching bag.

Unhappy with CBO’s reports estimating their GOP health bills would leave tens of millions more people uninsured, top Republican lawmakers have taken to disregarding CBO’s analyses altogether, picking apart its methodology and asserting bias.

Now eight former CBO directors are hitting back with a letter to congressional leadership, making the case for CBO’s importance.

“We write to express our strong objection to recent attacks on the integrity and professionalism of the agency and on the agency’s role in the legislative process,” they wrote. “As the House and Senate consider potential policy changes this year and in the years ahead, we urge you to maintain and respect the Congress’s decades-long reliance on CBO’s estimates in developing and scoring bills.”

The letter comes on the heels of two CBO reports released this week on the Senate’s revised health bill, which is estimated to insure 22 million fewer Americans than would be insured under Obamacare, and a bill to repeal large parts of the Affordable Care Act, which would insure an estimated 32 million fewer people.

House Speaker Paul Ryan (R-WI) called their most recent projections on the revised Senate health bill “bogus.” Sens. Bill Cassidy (R-LA) and Lindsey Graham (R-SC) shared a similar sentiment, poking holes in the CBO’s methodology in an effort to disregard the projections altogether.

Rep. Tom Garrett (R-VA) even accused the CBO of conspiring against Congressional Republicans, saying the office purposefully released an updated negative report of the Obamacare repeal bill on the same day the House’s archconservatives filed a motion to force a floor vote on the bill. Others, like Rep. Tom MacArthur (R-NJ), point to the CBO’s projections for Obamacare back when the law was passed, which overestimated how many people would be insured.

The CBO has been off in its projections in the past (although its original Obamacare analysis was still closer than most competitors). And it’s still the only official and nonpartisan estimation of the effects of legislation, former CBO directors argued:

“CBO’s approach produces consistent comparisons of competing legislative proposals and unbiased projections of the impact of policy changes,” the letter said. “Unfortunately, even nonpartisan and high-quality analysis cannot always generate accurate estimates.”

As Republicans barrel toward some health care-related vote next week, it’s becoming more and more apparent that the GOP’s main defense against projections of insurance coverage losses is just to claim, sometimes only on intuition, that it won’t happen.

Here is the letter in full: