WASHINGTON — The Trump administration announced on Tuesday that it was slashing grants to nonprofit organizations that help people obtain health insurance under the Affordable Care Act, the latest step in an escalating attack on the law that threatens to destabilize its insurance markets.

The cuts are the second round in two years. The government will provide $10 million this fall, down from $36 million last autumn and $63 million in late 2016 — a total reduction of more than 80 percent.

Trump administration officials said the insurance counselors, known as navigators, did not enroll enough people to justify more spending. Insurance agents and brokers do much better, they said.

The announcement on Tuesday, by Seema Verma, the administrator of the Centers for Medicare and Medicaid Services, came three days after the administration suspended a program that stabilizes health insurance markets by paying billions of dollars to insurers that enroll large numbers of unhealthy people under the Affordable Care Act. Insurers said the freeze would cause turmoil in insurance markets and drive up premiums.