In filing the brief, the administration abandoned an earlier position — that some portions of the law, including the provision allowing states to expand their Medicaid programs, should stand. The switch, which the administration disclosed in late March, has confounded many people in Washington, even within the Republican Party, who came to realize that health insurance and a commitment to protecting the A.C.A. were among the main issues that propelled Democrats to a majority in the House of Representatives last fall.

The filing was made in a case challenging the law brought by Ken Paxton, the attorney general of Texas, and 17 other Republican-led states. In December, a federal judge from the Northern District of Texas, Reed O’Connor, ruled that the law was unconstitutional.