The Voting Rights Act was passed by Congress and signed into law by President Lyndon B. Johnson at the height of the civil rights era. In the Shelby County decision, Chief Justice John G. Roberts Jr. wrote that “our country has changed” since those days of rampant voter discrimination.

The decision — which freed nine states, mostly in the South, to change their election laws without advance federal approval — unleashed a wave of new voting arrangements in scores of jurisdictions throughout the country. Civil rights advocates argue that those changes, including voter identification requirements that have since been overturned, have been intended to make it more difficult for blacks, Hispanics and other minorities to vote.

The Shelby ruling did not specifically address the Justice Department’s authority to send observers inside polling places. But Ms. Gupta said department lawyers had interpreted the decision to mean that officials could send observers only into jurisdictions where there was already a relevant court order regarding voting practices.

She said department lawyers felt bound to limit their use of poll observers because the program had used the same “coverage formula” that the Supreme Court had invalidated as a basis for requiring certain jurisdictions to get pre-approval for changing election rules.

That broad interpretation has puzzled some legal scholars on both the left and the right.

“I was a little surprised by the Justice Department’s decision, to be honest,” Derek T. Muller, a conservative scholar on election law at Pepperdine University School of Law, said.

Mr. Muller said he believed that the Justice Department could legally send observers to jurisdictions not specifically covered by court orders. “Until a court tells them otherwise, that provision is still on the books,” he said.

In any case, the restrictive new voting procedures enacted after the Shelby ruling threaten to have a chilling effect on minorities, particularly if Mr. Trump’s supporters show up at polling places to challenge voters’ status, said Nina Perales, the litigation director for Maldef, a Hispanic civil rights group.