WASHINGTON—A fast-approaching deadline for insurers to commit to selling health plans next year under the Affordable Care Act is pressuring Republican lawmakers to decide quickly whether to shore up the law and ease the path for insurers or continue efforts to roll it back.

Lawmakers returning to the Capitol from recess on Sept. 5 will have only 12 legislative days to decide whether to pass a bipartisan bill aimed at bolstering the ACA’s markets before insurers must commit to participating in the law’s exchanges in 2018. At the same time, a plan from Sens. Lindsey Graham (R., S.C.) and Bill Cassidy (R., La.) that would largely topple most of the ACA is gaining traction among Republicans.

The looming deadline means that Republican lawmakers who have been bogged down for months on legislation to rework most of the ACA will have little time to decide whether to pivot and instead help bolster the current health law—or, possibly, to pursue both courses.

A bipartisan plan from Sens. Lamar Alexander (R., Tenn.), chairman of the Senate’s health committee, and Patty Murray (D., Wash.), the committee’s top Democrat, would need support from senators in both parties to clear a 60-vote threshold in the Senate. Hearings are slated for the first two weeks after Congress returns.

Their proposal would likely preserve for next year billions of dollars in federal payments to insurers known as cost-sharing reduction subsidies. Insurers have said that without the payments they likely would raise premiums or stop participating on the ACA’s individual markets. In return for guaranteeing the payments next year, any bill would likely give states more flexibility on ACA implementation, a change GOP lawmakers have sought.