Two days before his 91st birthday last month, Democrat Robert C. Byrd of West Virginia was re-elected president pro tempore of the Senate.

He had recently decided to step down from his post as chairman of the Appropriations Committee, a move hastened by not-so-subtle pressure from a Democratic leadership clearly concerned about his capacity to fulfill the duties of one of the toughest legislative jobs on Capitol Hill.

The office of the president pro tempore, established under the Constitution and typically awarded to the senior senator from the majority party, has few responsibilities, formal or informal — unless catastrophe strikes and its occupant is called on to become president of the United States.

By virtue of the 1947 presidential succession law, Byrd stands third in line to the presidency, behind the vice president and the Speaker of the House.

He is no longer the adept master of Senate rules and folkways who outwitted adversaries and rose to the rank of majority leader more than three decades ago. He gets around by wheelchair and his speeches on the Senate floor are delivered with greater difficulty and less lucidity than ever before.

If Byrd is no longer capable of running the Appropriations Committee, it is “very questionable” whether he should be in line to run the country in a national emergency, according to Joel K. Goldstein, a constitutional law expert who teaches at Saint Louis University’s law school.

“It really doesn’t make any sense to have somebody whose only claim to the position is seniority a few heartbeats away from the presidency,” Goldstein said. “It’s very hard to justify.”

A Democratic leadership aide confirmed that there have been past discussions about whether Byrd should continue in the post of president pro tempore.

But Byrd’s recent re-election indicates that talk has faded about taking away his last major vestige of privilege in an institution he has served in since 1959 — before seven members of the Senate in the 111th Congress were born.

There is nothing unusual about an old — or infirm — man standing so close in line to the Oval Office.

When Lyndon B. Johnson ascended to the presidency in the wake of President John F. Kennedy’s 1963 assassination, there was no provision for selecting a new vice president. House Speaker John McCormack of Massachusetts, about to turn 72, and Senate president pro tempore Carl Hayden of Arizona, then 86, were next in line.

One of Byrd’s recent predecessors, the late Democrat-turned-Dixiecrat-turned-Republican Strom Thurmond of South Carolina, was 98 when he last held the job in summer 2001. Thurmond lost the post when then-Republican Jim Jeffords of Vermont became an independent, throwing control of the chamber to Democrats.

By that point, Thurmond was no longer performing his basic duties, he needed aides to help him around the Capitol and on the floor of the Senate, and he would soon change his address to Walter Reed Army Medical Center.