Two months later, Texas and the 19 other states filed suit in the Federal District Court in Fort Worth, asserting that the mandate could no longer be justified as a tax and should therefore be struck down — and arguing that as a result, the rest of the law must be invalidated, too.

The Justice Department brief did not go so far, but it said that Judge Reed O’Connor should void both the mandate and the protections for people with pre-existing medical conditions, while leaving in place the expansion of Medicaid in more than 30 states.

Attorney General Jeff Sessions said that it is “a rare case” for the Justice Department not to defend provisions of a law but added that he could not find any “reasonable arguments” to support their constitutionality.

If the administration prevails in the case, the full force of the decision would not hit until after the midterm elections on Nov. 6. But insurers said the legal debate alone could cause turmoil in insurance markets this summer.

“At the very least it adds uncertainty at exactly the moment when plans are trying to set rates for next year,” said Ceci Connolly, the chief executive of the Alliance of Community Health Plans. “At the worst it could strip away guaranteed coverage for those with pre-existing conditions.”

The main trade association for health insurers came out strongly against the administration’s position.

“Removing those provisions will result in renewed uncertainty in the individual market, create a patchwork of requirements in the states, cause rates to go even higher for older Americans and sicker patients, and make it challenging to introduce products and rates for 2019,” said Matt Eyles, the president and chief executive of America’s Health Insurance Plans, a trade group for insurers.