The payments - known as cost-sharing reduction payments - reimburse insurance companies for discounts on copayments and deductibles that they're required by law to offer to low-income customers.
But the administration has also missed self-declared deadlines for a decision. Trump still needs to commit to paying CSR payments in September; that payment is due September 20th.
The CBO, as an economic forecasting organization, treats the potential end of the health law's CSR payments primarily as a policy question, looking at the likely ripple effects on the insurance market as it exists under Obamacare.
Obamacare's exchanges are still fragile: Many insurers raised premiums and pulled out of some markets a year ago after finding the customers who signed up tended to be sicker and more expensive than they hoped.
This would not be severe enough, however, to cause the markets to collapse, the CBO found.
US markets close weak amid US-North Korea tensions
Penney slumped as much as 18.25 percent to a record low after the retailer reported a bigger-than-expected quarterly loss. Across Europe, Gemany's Dax was off 1.2% and the Cac 40 in France dropped by 0.6%.
"Try to wriggle out of his responsibilities as he might, the CBO report makes clear that if President Trump refuses to make these payments, he will be responsible for American families paying more for less care", the Senate Democratic leader, Chuck Schumer of NY, said.
According to the CBO, health insurance premiums for silver plans will climb 20% in 2018, one million more Americans would be left uninsured and about $194 billion would be added to the federal deficit if Trump does end the ACA subsidies.
"This disastrous law has devastated the middle class, and must be repealed and replaced".
The CBO estimate of the effect of ending the cost-sharing payments on premium hikes almost matches what independent nonpartisan experts previously predicted. Some insurers have pulled out of the individual market, notably Anthem Blue Cross.
"Regardless of what this flawed report says, Obamacare will continue to fail with or without a federal bailout", White House spokesman Ninio Fetalvo said in a statement. But the number of counties nationwide with no insurer serving the marketplace is now down to only two, from 38 earlier this summer, according to the Kaiser Family Foundation, which tracks the issue.
The cost-sharing reductions have always been a flash point. The ruling is on hold pending a decision on an appeal started by the Obama administration, which has been handed over to Trump's White House. The judge ruled that the president has an affirmative duty not to make the payments without an explicit Congressional appropriation, and that doing so violates the Constitution.
Officially, the administration remains undecided about how long it will continue making CSR payments, which are at the center of a federal court case that challenges their legality.