Anthem Inc. said that if it doesn’t quickly get more certainty about the future of the Affordable Care Act exchanges, it will likely further pull back its planned participation for next year, a threat that adds to the pressure on Senate Republicans as they struggle to pass health-care legislation.

The big insurer, speaking during its second-quarter earnings call, strongly emphasized that it needed answers about the future of federal payments that help reduce out-of-pocket costs for low-income ACA exchange-plan enrollees. Chief Executive Joseph R. Swedish said that without greater clarity, particularly around the cost-sharing payments, “we will need to revise our rate filings to further narrow our level of participation.” He added that the insurer may make decisions “in a relatively short period of time” and in September at the latest.