(CNN) Eight former directors of the Congressional Budget Office sent a letter to congressional leadership Friday to underscore the agency's importance and respond to criticism from Republican lawmakers and the White House.

"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," the letter said.

The nonpartisan agency has come under fire in recent months for its analyses of several versions of the Republican House and Senate health care bills.

Most recently, the CBO estimated Wednesday that the Senate bill to repeal but not replace the Affordable Care Act would leave 32 million more people uninsured by 2026 than under current law. A report Thursday estimated 22 million more would be uninsured under a full repeal and replace bill.

Republicans have been openly critical of the CBO's numbers. House Speaker Paul Ryan on Thursday called the CBO's estimate "bogus" and "not credible." Marc Short, the White House's director of legislative affairs, and National Economic Council aide Brian Blase published an op-ed in The Washington Post on Sunday calling the CBO's methodology "fundamentally flawed."

The White House even tweeted a video last week criticizing the CBO for inaccurately estimating ACA enrollment numbers. In May, White House Office of Management and Budget Director Mick Mulvaney questioned the agency's very existence, asking the Washington Examiner , "Has the day of the CBO come and gone?"

The Congressional Budget Office's math doesn't add up.



Faulty Numbers = Faulty Results pic.twitter.com/zdf4bZ01ma — The White House (@WhiteHouse) July 12, 2017

According to the signers of the letter to congressional leaders, the answer to Mulvaney's question is no: The CBO has served the American people well for 42 years, the former directors contend.

"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 said.

Ryan and Senate Majority Leader Mitch McConnell did not immediately respond to a request for comment.

The former directors include a number of scholars and executives of both conservative and liberal leanings: Dan Crippen, former executive director of the National Governors Association; Douglas Elmendorf, dean of the Kennedy school of Government at Harvard University; Douglas Holtz-Eakin, American Action Forum president; June O'Neill, Baruch College economics professor; Peter Orszag, managing director of Lazard; Rudolph Penner, Urban Institute fellow; Robert Reischauer, Urban Institute president emeritus; and Alice Rivlin, Brookings Institution senior fellow.

Current CBO Director Keith Hall, a former staff economist for President George W. Bush, was chosen by GOP leaders in 2015. He did not sign the letter.

While the signers acknowledged that not every CBO estimate is accurate, they argued that "such analysis does generate estimates that are more accurate, on average, than estimates or guesses by people who are not objective and not as well informed as CBO's analysts."

The letter comes as McConnell plans to hold at least one vote on a Republican health care plan next week.