After spending almost three weeks in and out of the hospital, Senate Appropriations Chairman Robert Byrd (D-W.Va.) returned to the chamber today to help his party cast close votes on budget resolution amendments, but that did not put to rest rumors about the 90-year-old’s continued fitness to serve in the top echelons of Senate leadership.

Indeed, Senate Democratic sources, most of whom declined to be quoted on the record for this story, said Byrd’s long absence has raised questions over whether he is physically able to continue as chairman of what is widely seen as the most powerful committee in Congress, as well as whether it is wise to keep such a frail, elderly man as President Pro Tem — a position third in the line of succession to the presidency.

During Byrd’s hospitalization, talk has abounded about the severity of his illness. Much of that speculation was fueled by the fact that he did not have contact with even senior Members of the leadership until he returned to the floor today in a wheelchair.

Senate Democratic sources indicated earlier this week that the party leadership would be evaluating Byrd’s condition over the upcoming two-week recess before deciding whether to consider replacing him in either his committee or leadership positions. But it was unclear, now that Byrd has returned, whether those evaluations would actually take place.

Asked this morning whether he had spoken to Byrd during his hospitalization, Senate Majority Leader Harry Reid (D-Nev.) said he had not, and Reid did not know whether Byrd would rejoin his colleagues just hours later.

Addressing the notion that some Democrats were agitating to replace Byrd as President Pro Tem and/or as Appropriations chairman, Reid declined to comment.

“I’m not getting into that. Unless I have talked to Senator Byrd, it would be terribly impolite of me to be talking to a reporter about him. I’m not going to do that,” Reid said.

Even Byrd’s reappearance was shrouded in mystery. His staff did not send out an e-mail about his release from the hospital or his presence on the Senate floor until the chamber had already begun voting on amendments to the budget resolution. That statement offered no remarks from the West Virginia Democrat, saying only: “Sen. Robert C. Byrd, (D-W.Va.) was released from the hospital late last night and is in his Capitol Hill office as the members of the Senate are casting key votes on the [fiscal] 2009 budget resolution. Byrd had been recuperating in the hospital as a result of a fall in his home, as well as suffering from a reaction to an antibiotic he was taking for an infection.”

Democratic Senators were told Wednesday that Byrd’s presence today was a possibility, but not a certainty. However, rumors about the longest-serving Senator in history even spanned to the other extreme — that his condition was so grave that Byrd would not return to the Senate at all.

If Byrd were unable to continue his Senate duties in any capacity, Reid likely would be put in the uncomfortable position of figuring out how best to move forward with his high-power positions. Next in seniority behind Byrd on the Appropriations panel is Sen. Daniel Inouye (D-Hawaii), 83, who would have the right of first refusal to take over, but would have to first abandon his gavel on the Commerce, Science and Transportation Committee. It is widely believed that Inouye may want to stay at Commerce, a move that could put Sen. Patrick Leahy (D-Vt.), chairman of the Judiciary panel, in line at Appropriations.

The President Pro Tem position is a largely ceremonial slot and typically is given to the most senior member of the majority party. Behind Byrd in seniority among Democrats is Sen. Edward Kennedy (Mass.).

According to Byrd’s office, the chairman fell at his home on Feb. 25, but was not admitted to the hospital until the next day. Although doctors determined he had not broken any bones, Byrd was kept in the hospital until Feb. 29.

On March 3, Byrd’s office announced that he was resting at home and taking antibiotics for a urinary tract infection, but he was readmitted to the hospital on March 5 to deal with an adverse reaction to the medication. He remained in the hospital until Wednesday night. He continues to undergo physical therapy as part of his recovery from the fall.

It was uncertain whether Byrd would be present for all the budget votes today in the daylong vote-a-rama, in which the Senate votes almost continuously for hours on end. Senate Budget Chairman Kent Conrad (D-N.D.) worried publicly this week that Byrd’s absence could imperil passage of the budget measure because of the Democrats’ narrow 51-49 majority. All three presidential contenders, Sens. Hillary Rodham Clinton (D-N.Y.), Barack Obama (D-Ill.) and John McCain (R-Ariz.) returned for the vote-a-rama, in which dozens of politically toxic issues are on deck.