No, not "largely responsible", but a factor. The big challenge for risk corridor funds is that they were designed to be funded Robin Hood - style: carriers selling on the exchange whose plans were profitable would contribute to risk corridor funding to cover losses from carriers whose plans were not profitable.
Due to lack of enrollment on the exchanges, and the enrollment of individuals who were, on average, "sicker" than the general population (adverse selection), the "risk corridor" pool was under-funded. In fact, 2014 collections totaled only $382 million -- nowhere near the $2.87 billion promised to insurers. As a result, Centers for Medicare and Medicaid Services (CMS) made good on only 12.6 percent of insurers' risk-corridor claims for the 2014 enrollment year.
It never got much better for the risk corridor program. The funding issues continued in subsequent years due to lack of enrollment and adverse selection. Also, a defunct insurer that tried to collect payments from the program was told by a
Federal judge to go pound sand, because the agreements made with the Federal gov't about collecting from the fund were deemed to be non-binding on Uncle Sam.
Rubio's move was intended to block what would have been essentially a tax payer bailout of insurance carriers by diverting federal funds to the risk corridor program. The Obama admin tried making Federal funds available from a
DOJ program to bail out the risk corridor program.
So, you folks who are castigating Rubio for blocking these funds are essentially pissed because Obama didn't use tax payer funds to bail out insurance companies. Imagine a bunch of lefties pissed about insurance companies not getting tax payer bailouts.