In another tweet on Saturday, Mr. Trump said he was “very proud” of an executive order he had signed Thursday. The order sought to increase competition among insurers, in large part by encouraging associations to offer less comprehensive plans and by allowing more short-term coverage.

The Congressional Budget Office has warned that stopping the subsidies is likely to raise insurance premiums, affect the stability of the health exchange markets set up under the law and, ultimately, add to the federal deficit.

On Friday, stocks for Centene, Molina and Anthem, insurers that have participated in state marketplaces under the Affordable Care Act, slumped by more than 3 percent. Health care providers were also affected. Shares in Acadia Healthcare declined by more than 3 percent and in Tenet Healthcare dropped by just over 5 percent.

While Mr. Trump appeared to celebrate this on Saturday, he has pointed to rising stocks as a sign of success in the past.