Former leaders of the Congressional Budget Office are defending the agency under scrutiny from the White House and Republican-controlled Congress.

Eight former directors of the CBO sent a letter to Republican and Democratic House and Senate leaders on Friday. The letter was in response to attacks from Republicans in Congress and the White House questioning the integrity of the agency that has given poor marks to the GOP's Obamacare repeal efforts.

"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 posted on Medium said.

The CBO predicted that the latest version of a Senate healthcare bill would result in 22 million people going without insurance over the next decade. The figures quickly drew a fresh round of recriminations from Republicans, who have been particularly critical of the amount of coverage that the CBO attributes to the individual mandate.

White House Budget Director Mick Mulvaney said that the score wasn't fair because it used methodology from former Obamacare architect Jon Gruber.

Mulvaney told the Washington Examiner back in May that the days of using the CBO to fairly score legislation "has come and gone." He also criticized Holly Harvey, an employee of the CBO, who scored the Obamacare bill and the GOP bill, saying that she was brought in by Democrats. Mulvaney later apologized for singling out Harvey.

The CBO directors said the agency has always been firmly committed to nonpartisan and high-quality analysis.

"To produce estimates of high quality, CBO uses its detailed understanding of federal programs and economic conditions, ongoing interactions with government officials and private-sector experts, the best academic research, and the latest available data consistent with the timing of the Congressional budget process," they wrote.

They added that every analysis "cannot always generate accurate estimates. Policy changes are often complex, the economy is dynamic and defies precise prediction, and many policies are modified over time."