Robert B. Morgan, a North Carolina Democrat who was a freshman US senator when he cast crucial votes in favor of treaties that transferred control of the Panama Canal to the Panamanian government, a decision that brought a swift end to his Senate career but which he stood by all his life, died July 16 at his home in Buies Creek, N.C. He was 90.

The cause was complications from Alzheimer’s disease, said his former Senate chief of staff, Carroll Leggett.

Mr. Morgan practiced law and ascended the ranks of North Carolina politics before his election to the US Senate in 1974. He served in the North Carolina state Senate, including a stint as president pro tempore, from 1955 to 1969 and later was state attorney general, developing a reputation as a hard-charging activist for consumer rights.

In the US Senate, he assumed the seat vacated by retiring Senator Sam J. Ervin Jr., who came to national attention as chairman of the Senate committee that investigated the Watergate scandal during the Nixon administration.

Mr. Morgan accumulated a voting record that ‘‘defies ideological labels,’’ according to the Almanac of American Politics. He was liberal on some issues but conservative on others, and he gained his greatest prominence on the matter of the Panama Canal.

The canal and surrounding area, a critical waterway that connects the Pacific and Atlantic oceans, had been controlled by the United States since 1903, an arrangement that by the 1970s had caused increasing friction with the Panamanians.

President Jimmy Carter, elected in 1976, became persuaded that authority over the canal should reside with the Panamanian government. Opponents of his position regarded any treaty to that effect as a ‘‘giveaway.’’

Mr. Morgan was initially among those opponents. He changed his position after visiting Panama as a member of the Senate Intelligence Committee and meeting with the CIA contingent there and with Panamanian leaders.

‘‘Our relationship with Panama on the future of the canal is a festering sore and affects our relations not only with Latin America but with the rest of the world,’’ the News and Observer of Raleigh, N.C., quoted Mr. Morgan as saying in a 1977 speech. ‘‘Our global position as world leader and a moral standard bearer is seriously weakened by maintaining this vestige of colonialism.’’

Two treaties were hammered out, one establishing the right of the US military to defend the canal’s neutrality and the other giving control of the canal to Panama by 1999.

Together, Mr. Morgan argued in comments reported by the Charlotte Observer, the treaties would ‘‘allow us to maintain our vital interests in that country while allowing the Panamanians the dignity and benefit of living on their own land — a fact we would surely insist upon in our part of the United States. It is just plain right to do so.’’

The treaties were signed in 1977 but faced withering opposition led in part by North Carolina’s senior senator, Jesse Helms. In 1978, the Senate ratified the treaties by 68 to 32 — just one vote more than the minimum required.

In 1980, Mr. Morgan was defeated by a relatively unknown law professor, Republican John P. East, who attracted the support of Helms’s political machine. In his campaign, East told voters that Morgan had ‘‘voted to give your Panama Canal away.’’

Mr. Morgan served in the Navy before receiving a bachelor’s degree from what is now East Carolina University in Greenville, N.C., in 1947 and a law degree from Wake Forest University in 1950.

He returned to the Navy to serve in the Korean War and remained in the Navy Reserve until 1971, attaining the rank of lieutenant commander. He later served in the Air Force Reserve, retiring as a lieutenant colonel.

In 1960, Mr. Morgan managed the unsuccessful gubernatorial campaign of I. Beverly Lake, a segregationist, who lost his bid for the Democratic nomination to Terry Sanford.

After his Senate tenure, Morgan ran the North Carolina State Bureau of Investigation and the North Carolina Center for Voter Education, an organization that worked on campaign finance issues.

‘‘I made a lot of decisions, and some cost me politically, cost me dearly,’’ Mr. Morgan told the Fayetteville (N.C.) Observer in 2012, looking back in particular on his votes on the Panama Canal treaties. ‘‘But they were decisions I made with a clear conscience.’’