Two lawmakers Lautenberg would normally work with said they hadn't spoken to him in several weeks, dealing instead with his staff.
Even as gun control moved to the front of the national agenda, Lautenberg made no floor speeches and held no public events to trumpet one of his most cherished issues. His name remained atop a bill to limit the size of gun magazines, but Sen. Richard Blumenthal (D., Conn.) led the losing fight for the ban.
"He's very much a presence here in spite of his physical absence. His past work, his spirit, and his intellectual contribution have been very instrumental," Blumenthal said last week. But as for dealing directly with Lautenberg, "he communicates with us through his staff. I haven't talked to him personally except when he was on the floor here."
U.S. Rep. Bill Pascrell (D., N.J.) said he last spoke to Lautenberg three weeks ago. He, too, has been in contact with the senator's staff since.
Lautenberg also missed a few weeks' work in December and January because of flu and bronchitis. The time he spent laid up brought on the muscular problems, but chemotherapy treatments he received for stomach cancer three years ago may have contributed, an aide said.
In January, Lautenberg was using a cane. Now he is in a wheelchair.
"Sen. Lautenberg has been rehabbing and is rebuilding strength in his legs. He is fully expected to return to Washington on a regular basis," said spokesman Caley Gray.
But it's still unclear when Lautenberg will be back, and some in Washington have questioned if he will ever again be a regular on the Senate floor.
He isn't seeking reelection next year. Gray has said the senator intends to finish his term, which ends in January 2015.
To be sure, many votes he has missed were so lopsided that Lautenberg could not have changed the outcome, but he missed some major showdowns: votes on a relief package for Hurricane Sandy, the "fiscal cliff" drama, and the Democratic budget, approved with the bare-minimum 50 votes.
Lautenberg has been in "constant contact" with staff, other senators and Democratic leaders, Gray said, noting that he returned when gun votes were taken and his vote was needed.
On his way out of the Capitol that day, Lautenberg, his speech slowed, delivered a muddled verdict on the results, which most saw as a crushing defeat for Democrats: "On a couple of them, they were the way I voted, but we lost some, and that was not as good as I like."
To be fair, supporters say he has always rambled a bit, even on his best days. His official statement, released that night, was clearer: "Republicans have no substantive reasons to oppose this moderate proposal that included real concessions on each side."
An aide wheeled him from the building that day and helped him into a silver Infiniti SUV. Lautenberg hasn't been back since.
He is more than a decade older than contemporaries who have recently announced plans to retire, including Jay Rockefeller, 75, and Tom Harkin, 73. Montana's Max Baucus, 71, was the latest, saying he won't run again in 2014. "I don't want to die here with my boots on," Baucus told the Associated Press last week.
By contrast, when Lautenberg was 78, in 2002, he was gearing up for a second Senate stint. He had tried retirement and hated it.
The Senate has often seen ailing members forced to the sidelines. But in a white-haired chamber where a youth infusion has brought the average age down to 62, such topics are taboo. Aging, ailing senators are afforded space and time, even when questions arise about how involved they truly are. Edward M. Kennedy and Robert C. Byrd died in office after extended illnesses and long absences. Strom Thurmond set the record by serving to 100.
Lautenberg's office has issued a steady flow of news releases and Twitter posts trumpeting his priorities. Meanwhile, his YouTube page, showing videos of hearings and speeches, hasn't been updated since Dec. 5.
For Democrats, any private belief that he should retire is tempered by political reality: Gov. Christie, a Republican, would choose a replacement to fill what is normally a safe Democratic seat.
Democrats hold a 55-45 Senate edge, counting independents who caucus with them.
Lautenberg "knows what's at stake," Pascrell said. "He's a fighter, and he never, ever, ever surrenders, so I have no real reason to believe that he can't see this through."
If he can't, a key date arrives in late August. A vacancy occurring by then would trigger a special Senate election in New Jersey this fall, giving a GOP replacement only a few months in office before Democrats got a chance to snatch the seat back.
After August, the election would be pushed to 2014, potentially giving a Christie appointee more time as an incumbent and altering the Senate's balance for a longer stretch.
No one who knows Lautenberg expects him to willingly leave the job that is so intertwined with his identity. His nearly 30-year Senate career is a point of immense pride for this son of Ellis Island immigrants.
"Will he make every vote?" Pascrell asked. "Obviously not, but he can finish this."
Contact Jonathan Tamari at firstname.lastname@example.org or follow on Twitter @JonathanTamari. Read his blog 'Capitol Inq' at www.philly.com/CapitolInq.