WASHINGTON (Reuters) - Aides to U.S. President Donald Trump on Sunday attacked the credibility of the nonpartisan agency that will analyze the costs of a replacement for Obamacare, as the White House sought to quell opposition from many conservative Republicans.

The Congressional Budget Office, which provides official estimates of the budget impact of proposed legislation, is expected to issue a report as soon as Monday that will assess the healthcare legislation put forward by Republican House of Representatives leaders.

The report could influence sentiment toward a bill already under fire from Democrats and some Republicans, especially if it suggests the legislation would reduce the number of Americans with health coverage or worsen U.S. budget deficits.

Republicans have long opposed Obamacare, formally called the Affordable Care Act, on the grounds it was government overreach and led to higher insurance premiums. The 2010 law, Democratic President Barack Obama’s signature legislation, provided 20 million previously uninsured Americans with health coverage.

Trump has called Obamacare a “disaster” and made its repeal and replacement a key campaign pledge.

Several Republican lawmakers said on Sunday the replacement bill was unacceptable in its current form, including conservative Senator Tom Cotton of Arkansas, who said the plan could not pass the Senate and could put the Republican House majority at risk in 2018 congressional elections.

“I believe it would have adverse consequences for millions of Americans and it wouldn’t deliver on our promises to reduce the cost of health insurance for Americans,” Cotton said.

In a series of television interviews, White House budget director Mick Mulvaney and top White House economic adviser Gary Cohn said the CBO was focusing on the wrong metrics with the estimates it will provide on the number of people who are insured. Cohn and Mulvaney said the CBO should instead should analyze whether patients can actually afford to go to a doctor.

“I love the folks at the CBO, they work really hard, they do, but sometimes we ask them to do stuff they’re not capable of doing, and estimating the impact of a bill of this size probably isn’t the - isn’t the best use of their time,” Mulvaney told ABC’s “This Week” program.

Speaking on Fox News, Gary Cohn, director of the White House National Economic Council, said: “We will see what the score is, in fact in the past, the CBO score has really been meaningless.”

“They’ve said that many more people will be insured than are actually insured. But when we get the CBO score, we’ll deal with that,” Cohn said.

‘COWARDLY’

The Trump administration’s criticisms of the CBO are unusual. Prior administrations, both Republican and Democratic, steered clear of attacking the credibility of the agency, which many lawmakers regard as a neutral arbiter. The CBO’s current director, Keith Hall, was appointed by Republicans in 2015.

White House Office of Management and Budget (OMB) Director Mick Mulvaney speaks with reporters during the daily press briefing at the White House in Washington, U.S., February 27, 2017. REUTERS/Jonathan Ernst

The credit rating agency Standard & Poor’s has estimated 6 million to 10 million people could lose health insurance coverage under the Republican plan.

Senator Bernie Sanders, who ran for president in 2016 as a Democrat, said it was “cowardly” for Republicans to proceed with a healthcare bill without CBO estimates, telling CBS’ Face the Nation show: “This is a disgrace.”

In recent weeks, Trump administration officials and Republican lawmakers have criticized what they said were overly optimistic estimates from the CBO of the number of Americans who would sign up for health insurance on government-run exchanges.

The CBO estimated in 2013 that 22 million people would be purchasing insurance through the exchanges in 2016. Only 10.4 million were signed up for those plans by the middle of last year, according to Department of Health and Human Services data.

The House Republican legislation would scrap tax penalties for Americans without health insurance, roll back an expansion of Medicaid insurance for the poor, and replace Obamacare’s income-based subsidies with a system of fixed tax credits to help people buy private insurance on the open market.

Cohn also disputed the notion that millions of people on Medicaid would become uninsured as Obamacare’s expansion of the program is rolled back over a period of years. He said many of these people would be transitioned into new private and employer-sponsored plans that would become more affordable under the Republican plan.

House Speaker Paul Ryan, the Republican plan’s top backer in Congress, said he was “certain” the CBO would show a reduction in the number of Americans with coverage.

“You know why? Because this isn’t a government mandate,” Ryan told NBC’s Meet the Press.

Conservative Republicans said, however, they could not support the plan without significant changes. Republican Representative Jim Jordan of Ohio, a founder of the conservative House Freedom Caucus, said it did not go far enough to meet Republicans’ promise to kill Obamacare.

“We told them we were going to replace it with something that would bring down the cost of insurance. That’s what we told them,” Jordan told Fox News Sunday. “This legislation that the speaker’s brought forward doesn’t do that.”