Washington (CNN) -- Colleagues of Sen. Robert Byrd of West Virginia mourned his death as family and friends planned his funeral. Byrd, the longest-serving member of the U.S. Congress, died Monday at the age of 92. Under West Virginia law, the state's popular two-term Democratic governor, Joe Manchin, has the power to appoint Byrd's successor. Manchin is expected to name a fellow member of his party to succeed Byrd, who was also a Democrat, thereby keeping a total of 59 Democrats in the Senate. There are questions, however, regarding exactly how long Byrd's appointed successor can serve before another election is held.
@highlight
NEW: President Obama: Senate has "lost a venerable institution"
@highlight
Questions raised about if/when a special election is required
@highlight
Harry Reid, Senate Democratic leader: "There will never be another like him"
@highlight
Void "can never be filled," says Sen. Jay Rockefeller, D-West Virginia
Neither @placeholder nor the Democratic-led Senate have made any official declarations yet.