WASHINGTON — The Trump administration Thursday announced proposed rule changes that would lead to a modest premium increase next year under the Affordable Care Act, potentially handing Democrats a new presidential-year health care issue.

The roughly 1 percent increase could feed into the Democratic argument that the Trump administration is trying to “sabotage” coverage for millions. The administration said the proposal is intended to improve the accuracy of a complex formula that affects what consumers pay for their premiums.

Premiums under the health law were basically stable this year after several sharp annual hikes. President Trump, who once predicted “Obamacare” would “implode” or “explode,” took credit for calm insurance markets. He said it reflects his administration’s management skills. In his first year in office, Trump tried unsuccessfully to repeal the health law and then rescinded a major insurer subsidy, triggering a wave of premium increases.

The latest details were in a 300-page proposed regulation released Thursday by the Centers for Medicare and Medicaid Services.

The agency said the reason for the change is to more accurately calculate premium subsidies under the law.

The proposed regulation is open for public comment for 30 days.