(CNN) - The Obama administration appears to be scaling back its expectations for enrollment of young adults under the Affordable Care Act.
For months, administration officials embraced estimates by the Congressional Budget Office anticipating 2.7 million of the 7 million enrollees under Obamacare by the end of March would be people between the ages of 18 and 34, or nearly 40 % of the total.
The overall metric is critical for making the program work economically. The larger the pool of healthy, younger people paying premiums, the lower the cost of providing coverage for older, sicker enrollees.
Asked by reporters about the 40% threshold, White House spokesman Jay Carney said the figure applies to the number of "uninsured who are young people."
"That's a little different from what you would need for the exchanges to have the demographic mix that's necessary for them to function effectively," Carney said.
Referring to the latest figures from the Department of Health and Human Services, Carney said youth enrollment currently stands at 27 percent.
"That is entirely consistent with where Massachusetts was," Carney said, pointing that state's health care system that served as the prototype for Obamacare.
Asked about Carney's comments, a White House official cited a different metric from Massachusetts as a potential milestone.
"If you look at (Massachusetts), they never got above 34 percent in any given month as to the share of enrollees who were young adults and that was considered a successful risk pool," the official said.
A spokesman for House Speaker John Boehner, a chief critic of Obamacare, accused the White House of trying to move the goal posts.
"Obamacare enrollment goals are melting away as quickly as the law’s deadlines. Youth enrollment has been a bust so far, and trying to whitewash stated benchmarks won’t change that," Brendan Buck said.
Buck referred to an article in The Atlantic, which cited a Powerpoint presentation by administration officials using the CBO's youth enrollment figures.
"That is/was/has been the target they set," Buck said.
White House officials note younger enrollment has accelerated in recent weeks, arguing that will improve the program's viability over time.
The administration has tried to lower expectations for this key metric for several weeks.
At a background briefing with reporters in January, administration officials argued Obamacare does not need to reach enrollment of 2.7 million young adults by the end of March to achieve a "sustainable market."