There are no hard-and-fast rules at the state or federal level about whether defense of a statute can be dropped, officials say. State attorneys general, like most public employees, swear an oath to support the U.S. Constitution, which is supreme to any state law or constitution. They regularly advise lawmakers when state legislation could be vulnerable to constitutional challenge, such as when it encroaches on congressional authority over interstate commerce.

In a 1980 letter to Congress, U.S. Attorney General Benjamin Civiletti wrote that when an apparent conflict arises between a statute and the Constitution, “it is almost always the case that [the attorney general] can best discharge the responsibilities of his office by defending and enforcing the Act of Congress.” But in “rare cases,” he added, “the Executive’s refusal to defend and enforce an unconstitutional statute is authorized and lawful.”…

Mr. Tierney worries, however, that such exceptions may become more commonplace in today’s heated political environment.

“It’s not just same-sex marriage. It’s abortion. It’s the Voting Rights Act,” said Mr. Tierney, mentioning other issues where attorneys general are likely to face strong political pressure on whether to defend state laws.