It's more than just a rule change: The so-called "nuclear option" will fundamentally alter the way the Senate operates - for good. (Casey Capachi/(In Play))

The Senate filibuster is not a c onstitutional concept. It is a legislative delaying tactic using rules intended to protect the sanctity of debate in the Senate.

Any senator recognized to speak may do so for as long as he or she likes. Senators cannot be stopped, or even interrupted, without their consent. That means an opponent of legislation may hold the floor and “debate” for as long as he or she can manage, to prevent a vote on that legislation. The practice gives enormous leverage to parties in the minority when they want to defy and stymie the will of the majority.

Delaying tactics began almost immediately in the Senate. In 1789, opponents blocked a bill to place the nation’s capital on the Susquehanna River. But the first full-fledged Senate filibuster did not come until 1837, and it was not until 1917 that the Senate put in place a mechanism to end filibusters. Called cloture, from the French word meaning conclusion or ending, it allows a supermajority to end the debate. The rise in the number of cloture motions in recent years illustrates how filibusters went from a rare, arcane maneuver in the early 20th century to a common tool of obstruction used today by both sides.

● In 1841, the minority Democrats threatened to block a bank bill written by Henry Clay of Kentucky, who threatened to change Senate rules to allow the majority to close debate. He was rebuked for trying to tamper with the Senate’s right to unlimited debate.

● In 1917, with President Woodrow Wilson at war with the Senate about preparing the nation for war, the Senate adopted a rule to allow a two-thirds majority to vote an end to a filibuster. The first successful use came in 1919, when cloture was invoked to end a filibuster against the Treaty of Versailles. In the next 46 years, cloture was invoked four more times in the Senate.

Then-Senate Majority Leader Bill Frist (R-Tenn.), shakes hands with then-Senate Minority Leader Harry Reid (D-Nev.) outside Frist's office in the U.S. Capitol on May 23, 2005. In a dramatic reach across party lines, Senate centrists agreed on a compromise that cleared the way for confirmation of many of President George W. Bush's stalled judicial nominees, while leaving others in limbo and preserving venerable filibuster rules. (LAUREN VICTORIA BURKE/AP)

● In the 87th Congress in 1961 and 1962, four cloture motions were filed. Only one succeeded, but the ’60s brought a series of intense debates, including those over civil rights and the Vietnam War. That deepened the divisions in the country and made filibusters a handy tool for senators opposed to legislation of one kind or another.

● Two decades of increasing partisan clashes came to a head after George W. Bush was reelected in 2004. The Republican majority invoked cloture 68 times during that Congress. In May 2005, a bipartisan “Gang of 14” tried to work on the problem, striking a deal to advance several Bush judicial nominees. But things would only get worse.

● By the 110th Congress, in 2007 and 2008, the last two years of the Bush administration, Democrats had taken control of the Senate, and the number of cloture votes, many related to the war in Iraq, climbed to a record 139. The first two years of the Obama administration remains a close second, with 137 motions filed.

● In February 2010, Senate Majority Leader Harry M. Reid (D-Nev.) dismissed an effort by some frustrated Democrats to eliminate the filibuster, saying the chamber’s procedures were designed to prevent the majority party from unilaterally changing the rules. Some junior Senate Democrats were pushing to change the rules to stop GOP attempts to block nominations and legislation.

● In January 2011, the Senate adopted the most significant rules change in 35 years by agreeing to limit the use of the filibuster and dropping confirmation requirements for about 400 federal agency nominees.

● The summer of 2013 brought another near-meltdown when Republicans agreed to confirm several of President Obama’s executive branch nominees in exchange for Democrats agreeing to leave existing filibuster rules in place. They did, until Thursday.

● Nov. 21, 2013: Senate Democrats voted to change the chamber’s rules so that federal judicial nominees and executive branch appointees can be confirmed by a simple majority of senators, rather than the 60-vote supermajority that has been required since 1975.

— From staff reports