The House of Representatives confirmed Mr. Pierluisi, 60, as secretary of state on Friday afternoon. But the Senate has yet to take up his nomination, leaving the legality of the appointment unclear. At least one senator has said he intends to challenge the process in court on Monday.

Puerto Rico’s Constitution requires that a secretary of state be confirmed by both chambers of the Legislative Assembly. But Mr. Rosselló availed himself of a legal statute from 1952 that was amended in 2005 to exempt the secretary of state — but no other official in the line of succession — from legislative confirmation.

Most legal experts on the island consider the amended statute to be unconstitutional. But because it has never been challenged, it must be presumed valid until a court rules otherwise, said Carlos E. Ramos González, a law professor at the Interamerican University of Puerto Rico.

“It’s absurd,” he said. “How is it possible, if the successor were the secretary of treasury or justice, to have to comply with all the constitutional requirements of confirmation, but not the secretary of state?”

The Senate could make any legal challenge moot if it confirms Mr. Pierluisi before the courts take up the issue, Mr. Ramos González said. The Senate is scheduled to vote in a special session on Monday.

Senator Eduardo Bhatia of the opposition Popular Democratic Party said he was preparing to sue.

“This is kindergarten in terms of leadership,” Mr. Bhatia said. “There is no way in hell someone like him, a secretary of state, can become governor unless he has been approved by both chambers.”

Mr. Rosselló was forced to resign last week following intense protests prompted by the publication of hundreds of mean and profane messages that the governor and members of his inner circle exchanged in a private group chat on the Telegram messaging app. The leaked chat transcripts unleashed a torrent of pent-up anger in Puerto Rico over financial mismanagement, broken promises and corruption.