Democrats also intend to convene hearings to spotlight the sweeping impact of the Texas ruling — and lay the groundwork for their case to reclaim the White House in 2020. If upheld on appeal, the decision could deprive an estimated 17 million Americans of their health insurance — including millions who gained coverage through the law’s expansion of Medicaid. Still others could see premiums skyrocket as price protections for pre-existing conditions lapse.

The immediate practical effect of the ruling was not clear. While the judge declared that the whole law was invalid, as Texas and 19 other states had asserted, he did not issue an injunction to stop federal officials from enforcing it, and the effects of the judgment could be delayed pending appeals.

But the timing of the decision seemed designed to maximize political reverberations. It came a day before open enrollment was to end for coverage under the health law for the coming year. Sign-ups were already expected to dip after successive blows to the law by Congress and the Trump administration. But the passage of statewide referendums last month and the election of Democratic governors could also mean coverage expansions in the coming year under the health law’s Medicaid expansion.

In an email to millions of Americans on Saturday, the Trump administration tried to allay concerns caused by the court decision in the Texas case.

The case is “still moving through the courts,” said the message from the federal Centers for Medicare and Medicaid Services. “The marketplaces are still open for business, and we will continue with open enrollment. There will be no impact to enrollees’ current coverage or their coverage in a 2019 plan.”

But Mr. Trump was in a celebratory mood. “It was a big, big victory by a highly respected judge, highly, highly respected in Texas, and on the assumption that the Supreme Court upholds, we will get great, great health care for our people,” Mr. Trump told reporters on Saturday. “We’ll have to sit down with the Democrats to do it, but I’m sure they want to do it also.”