While some cancellation notices already have gone out, insurers say the bulk of the letters will be sent in October, shortly before the next open-enrollment period begins. The timing — right before the midterm elections — could be difficult for Democrats who are already fending off Republican attacks about the Affordable Care Act and its troubled rollout.

Some of the small-business cancellations are occurring because the policies don’t meet the law’s basic coverage requirements. But many are related only indirectly to the law; insurers are trying to move customers to new plans designed to offset the financial and administrative risks associated with the health-care overhaul. As part of that, they are consolidating their plan offerings to maximize profits and streamline how they manage them.

“If they do it one way, the word canceled gets attached to it. If they do it another way, they say they are amending the policy. It sounds more gentle but it’s the same thing,” said Gary Claxton, an expert in private insurance at the Kaiser Family Foundation. “The basic point is, for many people in the small-group market at some point soon their coverage is going to change.”