For all the excitement in the Democratic Party these days, Senate Majority Leader Harry Reid (D-Nev.) has a dark cloud on his horizon: He must decide whether to supplant two of his powerful chairmen — who are both his friends — to ensure passage of an ambitious Democratic agenda in a new Congress.

Reid is under pressure from his rank and file to strip the gavels from Appropriations Chairman Robert Byrd (D-W.Va.) and Homeland Security and Governmental Affairs Chairman Joe Lieberman (ID-Conn.). Byrd, who will turn 91 next month, has been physically unable to perform some duties associated with doling out nearly a third of the federal budget, while Lieberman’s decisions not just to endorse Sen. John McCain (R-Ariz.) for president but also to go on the attack against Democratic nominee Sen. Barack Obama (Ill.) has made him a turncoat in many Democrats’ eyes.

Of course, a decision to replace Byrd comes with much more heartache for Democrats than a decision to oust Lieberman. Given Byrd’s revered status in the caucus, Democratic Senators and aides stressed that should Byrd step down from atop the Appropriations panel, it would have to be at least in part his decision. Democrats said they are committed to making sure he continues to hold a venerated role in the caucus no matter what happens.

Reid spokesman Jim Manley downplayed the notion that Reid was contemplating Byrd’s fate as chairman, saying Reid spoke to Byrd just two weeks ago to discuss the next steps on any economic stimulus measure the Senate takes up during the lame-duck session.

“As far as Sen. Reid is concerned, Sen. Byrd is an outstanding chairman who’s fought for years to improve the lives of the people of West Virginia as well as the United States,” Manley said.

Byrd also tamped down speculation that he might be considering surrendering the gavel. Through a spokesman, Byrd said, “I get along with soon-to-be-President Barack Obama just fine, and I look forward to working with him as chairman.”

As for Lieberman, Manley acknowledged that the Democratic caucus might take up the question of whether to strip him of his chairmanship following the election, and Lieberman appears to be readying himself, saying recently in an interview that he would not rule out switching parties in the future.

Both chairmen’s tenures probably will be discussed during the lame-duck session, set to begin Nov. 17. Aides said the matter is too delicate to discuss at lunches or other open forums. Rather, Reid probably will discuss it with Members — either at their insistence or his — to gauge the support for making changes atop both panels. A decision could come in December, several aides predicted.

Most Senators and aides approached for this story declined to be quoted, given the sensitivity of the issues.

Any change at Appropriations would take a diplomat’s touch to ensure any transition for Byrd is pulled off with dignity and respect.

Earlier this year, Democrats whispered about Byrd’s infirmities, concerned that a muscular Appropriations Committee would be needed to deal with the House and a new president.

“So many people have said to [Reid] that we can’t have a chairman this weak,” one senior Senate Democratic aide said.

But the aide noted that Reid cannot make the call in a vacuum: “There’s got to be enough critical mass of backing to make a decision.”

Democrats say Byrd’s inability to attend bicameral appropriations meetings puts the Senate at a disadvantage in negotiations with the House and its irascible Appropriations Chairman David Obey (D-Wis.). Byrd has regularly dispatched staffers to stand in for him during top-level leadership meetings on spending bills.

Byrd also has ceded most floor management duties to Democratic Conference Secretary Patty Murray (Wash.), who also sits on the panel.

Byrd was in and out of the hospital in February and March after a fall and a urinary tract infection, spurring leaders and rank and file to talk more openly about it. After reports in April that Democratic leaders discussed his ouster at their weekly meeting, Byrd made a round of phone calls to rank-and-file Senators for support and vowing to re-engage. He responded with several high-profile hearings.

Since then, his public appearances have raised questions about his health. At a 9/11 ceremony last month, House and Senate leaders appeared unnerved by Byrd’s repeated interruptions of their somber speeches with chants of, “Yes, Lord.” He has also been overheard cheering Senators’ speeches on the floor with what appear to be encouraging chants, such as “Yeah, man,” — statements that veer from traditional Senate etiquette.

Any scenario in which Byrd stepped down, aides said, would likely include assurances from Reid that he would retain his staff and Capitol office space. These aides noted that Byrd would also likely be named chairman emeritus or have some other venerable title to reflect his status as the longest-serving Senator.

A similar scenario has played out in the past with elderly or ill chairmen. Assistant Senate Historian Betty Koed said she is unaware of any situation in which the Senate has removed a chairman because of illness or age, and in the past 100 years, only three chairmen have voluntarily stepped down for those reasons — at least two with the subtle or overt encouragement of their party’s leaders.

The most recent example came in 1999, when then-Armed Services Chairman Strom Thurmond (R-S.C.) at age 96 relinquished his gavel to Sen. John Warner (R-Va.). Senators concerned about Thurmond’s age had been plotting for more than three years to make the change before it happened, allegedly with the help of then-Majority Leader Trent Lott (R-Miss.).

In 1959, former Senate Majority Leader Lyndon Johnson (D-Texas) helped engineer the resignation of 91-year-old Foreign Relations Chairman Theodore Green (D-R.I.), whose staff also pushed him to resign because of his confusion at hearings and his failing eyesight and hearing. Additionally, an ailing 79-year-old Senate Naval Affairs Chairman Carroll S. Page (R-Vt.) voluntarily surrendered his gavel in the 1920s.

Even if Byrd decided to step down or did so under caucus pressure, Democrats say they could end up exchanging one elderly chairman for another. Next in line to succeed Byrd is 84-year-old Sen. Daniel Inouye (D-Hawaii).

Some Democratic Senators and aides have expressed a belief that Inouye would prefer to keep his chairmanship of the less-prestigious Commerce, Science and Transportation Committee, but Inouye told the Washington Post in April that if Byrd voluntarily stepped aside he would expect to become chairman, based on the seniority system that Democrats have rarely, if ever, veered from. “No one would oppose it,” Inouye said at the time.

If Inouye declined to take the gavel, Sen. Patrick Leahy (D-Vt.) is next in line. He would have to relinquish his chairmanship of the Judiciary Committee, but he has long had ambitions for the Appropriations post. Murray appears to be the favorite of some in the caucus, but her promotion would be unprecedented and unlikely under seniority rules that govern the caucus. She would have to leapfrog over five more-senior members to take the post.

While Byrd’s future remains an open question, Lieberman’s ouster appears to be more likely, with Democrats wavering when the prospect of having a filibuster-proof majority arises. Should Democrats win enough seats in November to give them 60 Members, Lieberman could be more important as that 60th vote.

Some sources said even that might not save Lieberman. Most Democrats are talking about how forcefully to punish him. Talk has centered on whether to strip him of his chairmanship while allowing him to continue caucusing with Democrats or to kick him out of the party and strip him of his committee assignments.

Armed Services Chairman Carl Levin (D-Mich.) and Veterans’ Affairs Chairman Daniel Akaka (D-Hawaii) are next in seniority on the panel. If neither wanted to surrender their current panels, as is likely with Levin at least, Sen. Tom Carper (D-Del.) would be next in line.

Lieberman, who became an Independent in 2006 after losing his party’s primary to a more liberal Democrat, helped seal his fate when he gave a speech at the Republican National Convention that was highly critical of Obama, after having assured Democratic colleagues that he would limit himself to praise for McCain.

Since then, Lieberman has committed more perceived sins against the Democratic Party by writing an opinion piece defending endangered Republican incumbent Sen. Norm Coleman (Minn.). Earlier this month, Lieberman aggressively attacked Obama as not ready to be president in an interview with the conservative news service Newsmax. He also said that he would not rule out leaving the Democratic Party in the future.

“The Democratic Party of today is not the Democratic Party that I joined in the ’60s under my hero President Kennedy, and it’s not the Democratic Party of my dear friend Bill Clinton,” Lieberman told Newsmax.

Whatever happens, Senate Democrats said decisions on Byrd and Lieberman would not be decided until voters first determine the political fate of the country on Nov. 4.