He and others pointed to the fact that even though Congress erased the tax penalty, it did not touch the rest of the sprawling health act. A longstanding legal doctrine called “severability” holds that when a court excises one provision of a statute, it should leave the rest of the law in place unless Congress explicitly stated that the statute could not survive without that provision.

In this case Congress’s intention was particularly clear, legal experts said.

“Congress amended one provision of a 2,000 page law and did not touch the rest of the law so it is implausible to believe that Congress intended the rest of the law not to exist,” said Abbe R. Gluck, a health law expert at Yale Law School.

Judge O’ Connor also cited congressional intent, focusing on language from the 2010 law, which underscored the significance of the individual mandate to the entire act. But he largely ignored the 2017 congressional action. In essence, legal scholars said, he looked to one congressional view and not the more recent one.

And in so doing, he opened the door for House Democrats to intervene in successive appeals. On Saturday aides to Representative Nancy Pelosi, who is expected to become the next speaker of the House, said she would move quickly to notify the Trump administration that House Democrats intended to step in to defend the law in the case.

As the legal showdown plays out, efforts to protect the A.C.A. are also underway in the courts. Earlier this year the state attorney general of Maryland sued the Trump administration for attempting to gut the act. The case is pending.

Nicholas Bagley, a health law expert at the University of Michigan, suggested that Judge O’Connor may not yet be done with the case. In a series of tweets on Saturday, Mr. Bagley noted that the judge had not yet addressed a handful of central issues in the suit, nor had he issued a final ruling indicating whether the act should fall immediately. Judge O’Connor could indeed hold onto the case before an appellate court takes it up.