Sen. Robert Byrd (D, WV), the longest-serving member of Congress ever, died early Monday at age 92 of an undisclosed "serious" illness. Byrd was first elected to the House in 1952, and the Senate in 1958, and held just about every leadership post possible in his storied career. (Watch one of Robert Byrd's historic Senate moments.) Here's a brief look at what he accomplished and the potential political fallout from his death:

Who was Robert Byrd?
Born Cornelius Calvin Sale Jr. in 1917, Byrd went to live with his aunt and uncle, Vlurma and Titus Dalton Byrd, when his mother died in 1918. They adopted him and changed his name to Robert. He married a coal miner's daughter, Erma James, in 1937, and stayed married to her until her death in 2006. Largely self-educated, Byrd is the only member of Congress to work his way through law school while in office, from 1953-59, earning his law degree from American University in 1963. 

What's Byrd most remembered for?
His knowledge and spirited defense of the Senate as an institution, funneling federal money and projects to his home state, his lengthy filibuster of the 1964 Civil Rights Act and early membership in the Ku Klux Klan (both of which he later publicly regretted), his strong opposition to the Iraq War, and the stunning length of his service — in 2006, he beat Sen. Strom Thurmond's (R-SC) record in the Senate, and he topped Sen. Carl Hayden (D-AZ) as the longest-ever serving member of Congress on Nov. 18, 2009.

How will Byrd's seat be filled?
Byrd's replacement will be picked by Gov. Joe Manchin, a fellow Democrat. Under West Virginia law, Byrd's successor will serve until a special election this November if his seat is declared vacant before July 3; if it's declared vacant on July 3 or later, he or she will serve out the remainder of Byrd's term, which lasts until Jan. 3, 2013. 

Who might replace him?
Manchin has already ruled out appointing himself, but he is rumored to be interested in the seat. That could mean he'll name a placeholder until he makes a run for Senate in November, or in 2012, depending. U.S. Rep. Shelley Moore Capito is probably the strongest Republican if there's a special election in November.

What are the political ramifications of his death?
Most immediately, it means the Senate might delay a vote on the financial reform bill that Democrats had hoped to get to President Obama's desk by July 4. The bill barely passed the Senate, and some of the yes-voting senators are up in the air on the final bill melded in a House-Senate conference. In the longer term, Byrd's absence gives Republicans another shot to eat into, or even erase, the Democrats' majority in the Senate.

Sources: WBOY TV (NBC), FiveThirtyEight, Charleston Gazette, New York Times, Salon, Wall Street Journal, Michelle Malkin, Washington Post, Reuters