Why? A Republican win in the heavily Democratic state means Democrats would lose their 60-seat, filibuster-proof majority.

Prior to this Congress, the last time any party held a filibuster-proof majority in the U.S. Senate was during the 95th Congress from 1977 to 1979 when Democrats held 62 seats.

The current senate filibuster-proof majority will have lasted about seven months by the time Brown is sworn in.

The Senate requires 60 votes to invoke cloture, a parliamentary device used to stop a filibuster. The word "filibuster" comes from the Dutch word meaning "pirate." Now, it's a term used to describe any attempt to block or delay action on a bill by using procedures or obstructive actions, according to the U.S. Senate.

Unlimited debate was permitted in the Senate until 1917, when President Woodrow Wilson wanted the Senate to adopt a new rule: a two-thirds vote -- or 67 members -- to end a filibuster. But in 1975, the required vote count was reduced to three-fifths -- or 60 members.

Currently, Democrats have exactly 60 members in their caucus; Republicans have 40.

If the majority party needs 60 votes to pass a bill, and it can't win votes from the other side, a handful of moderates wield tremendous power.

A wild card in the Senate head count is Connecticut Sen. Joe Lieberman, an independent who has caucused with the Democrats but who was an ardent supporter of Republican presidential nominee Sen. John McCain in 2008. If Lieberman supports the GOP, Democrats cannot reach 60, and the filibuster remains in play.

The fight over health care reform demonstrates that 60 votes is now the minimum threshold for passing major legislation through the Senate.

Health care is not the only contentious issue that has put a spotlight on the filibuster procedure.

The Civil Rights Act of 1964 is often considered one of Congress' most influential pieces of legislation. The act banned discrimination based on race, color, religion, sex or national origin, among other things.

While the bill was intended to protect black Americans, it ultimately increased civil rights protection to all -- focusing heavily on women's rights in the workplace.

But the bill was met with heavy opposition, especially from Southern Democrats, otherwise known as Dixiecrats, as well as segregationists. The group tried to kill the bill with a filibuster, but there was enough Republican support to end the filibuster.

In the end, the act passed with overwhelming support.

Years before, another famous filibuster involved Sen. Strom Thurmond's stand against civil rights resulted in a 24-hour long monologue in 1957, which set the record for longest individual filibuster, Time Magazine notes.