It is the stuff of legend in Congress.

Each day, thousands of visitors file into the Capitol. Many sit in on House and Senate sessions from the public galleries. Some visitors aspire to be so lucky to catch a glimpse of a special, near-mythic, parliamentary spectacle.

Like a baseball fan lucky enough to watch a perfect game.

An ornithologist spying an Asian Crested Ibis.

Catching the transit of Venus in front of the sun.

This is a phenomenon most tourists would clamor to see. A tableau nearly synonymous with an elite guild of practitioners, populated with names like Thurmond, Long, Morse and D'Amato. That's to say nothing of their pop culture brethren "Jefferson Smith."

It is the Senate filibuster, among the most-fabled Congressional fauna.

And if Senate Majority Leader Harry Reid (D-NV) gets his way, the filibuster could find itself on the Capitol Hill endangered species list.

The Nevada Democrat is igniting a contretemps as he threatens to alter a fundamental Senate rule when the next Congress starts in January. In fact, detractors of Reid's plan characterize his effort in the most stark of terms. They've christened his scheme as the "nuclear option" - signaling the utter devastation it will inflict on the Senate, the impending radioactive fallout and the arrival of a legislative nuclear winter.

Reid doesn't want to get rid of the filibuster entirely. He just wants to curb some types of filibusters. He doesn't think lawmakers should be able to jam up bills from hitting the floor for debate and blocking final versions of legislation called "conference reports." And he also wants senators to take a page out of "Jefferson Smith's" playbook if they do intend to filibuster and actually monopolize the Senate floor like Jimmy Stewart's character did in "Mr. Smith Goes to Washington."

But few who pack the viewing galleries - let alone astute students of government, journalists and even some Congressional staff - realize that filibusters unfold almost every week in the Senate.

You just don't see them.

That's why Reid wants to limit them. And here's why:

The Senate is a fundamentally different body than the House of Representatives. The House is a larger, more unwieldy institution. It has a Rules Committee which governs which bills come to the floor, what amendments are in order and how much time is allocated for debate. House members are not equal. No one is guaranteed the right to speak or offer their amendments. The majority party rules and that's it.

It's different in the Senate. The Senate is a body of equals. Two members from each state no matter the size. The old joke is that there are only two "rules" in the Senate: exhaustion and unanimous consent. What that means is that the Senate can do practically anything once everyone is worn out. Everyone has spoken. Everyone wants to go home. Then senators can forge an agreement and finish a given bill. Unanimous consent is more complex (often known as "UC" in Congress-ese). Ninety-nine senators could be in favor of doing something. Passing a bill. Approving a nomination. Making Kim Kardashian's marriage last. But if there's not unanimous consent, the Senate can't even boil water.

And that is the essence of the filibuster and the fundamental glory of the Senate.

The term "filibuster" is derived from Dutch and Spanish. Someone who is a "filibuster" or "freebooter" is a pirate who tries to spur an uprising. When used in the Senate, it's typically a pejorative, meaning someone is fomenting dissent and tying the Senate in knots.

But are they really?

Senators may claim they're not filibustering, but simply exercising their rights. Or, they may argue what's the rush? The Senate is supposed to be the "deliberative" body. So, let us take time to debate.

But to Harry Reid, that's the problem. It's not that senators commandeer the floor for hours on end in vibrant arguments. It's that they don't debate. But they manage to gum up the works rather effectively by filibustering. But the filibuster is a far cry from the late-Sen. Strom Thurmond (R-SC) reading the phone book.

Here's what drives Reid up a wall:

Let's say Reid has a bill that he wants to move. So, he heads to the Senate floor to ask "unanimous consent" that they began debate on that measure. A fellow senator, usually a Republican, objects. Or better yet, as a senatorial courtesy, Senate Minority Leader Mitch McConnell (R-KY) notifies Reid in advance that Republicans plans to object. So have they called up the legislation in question for debate? No. Not if just one senator objected. Or intended to object. They couldn't get unanimous consent.

So the Senate is stuck.

Reid would call this a filibuster.

So what are Reid's options? Starting in 1917, the Senate altered its rules to get around such a filibuster. It crafted a proviso which enabled senators to "invoke cloture" to eventually break a filibuster. The caveat? Senators could still filibuster. But if 67 senators want to stop the filibuster, they had "invoked cloture" and the Senate could move things along. The Senate later modified the cloture rule to 60 senators to advance an issue in the 1970s.

So, if someone objects to Reid calling up a bill, Reid may file a cloture petition in an effort to get 60 votes. This is called the "motion to proceed" in the Senate. Once Reid files his petition, it must lay over for two nights. If he gets 60 votes two days later, the Senate can finally debate the bill.

But we're not done yet.

Say Reid wants to shut off debate? He has to file yet another cloture petition, wait an additional two nights for it to ripen and conjure up 60 votes again. Only then after having crossed the 60 vote threshold twice can Reid advance an issue to a final vote, which in most cases, only requires a simply majority of 51.

The "motion to proceed" is one of Reid's beefs. He suggests Republicans gum up the system by making him run the traps not once but twice to get to 60 votes. Moreover, rather than simply objecting on the floor or threatening they will object, he thinks they ought to pull a Jimmy Stewart. In addition, even if Reid beseeches 60 senators to vote yes on cloture, the timetable for simply bringing up a single bill, debating it and voting on it could consume the better part of a week or more.

Republicans contend one of the reasons they make Reid go the distance is because he often blocks them from offering amendments. In the Senate, there's something called "the amendment tree." Think of the legislation itself as the trunk of a tree. The tree has two branches. When the Senate's considering a bill, one amendment can go on one branch. A second amendment is slapped on the next branch. Over the past few Congresses, Reid and former Senate Majority Leaders Bill Frist (R-TN) and Trent Lott (R-MS) began to "fill the tree." It's a Senate tradition to recognize the Majority Leader first for offering amendments. If the leader fills the tree, no one else can offer amendments. The non-partisan Congressional Research Service says leaders of both parties have increasingly filled the tree to block other amendments.

Now why would a leader do that? Well, for a couple of reasons. First, the minority might try to file an amendment that, if adopted, would become a "poison pill" and potentially jeopardize the entire package. Also, the minority sometimes files amendments in an effort to make it challenging for lawmakers from the other side to vote yes on them. Other times, the amendments are a "dare" to vulnerable senators facing challenging re-election circumstances to vote against their state. That allows political operatives to then document that vote and use it against the senator in the next campaign. So Reid sometimes fills the tree to protect his exposed members.

Still, if Reid wants to change the rules, there's more to come.

A change to the standing rules of the Senate only takes a simple majority of 51. But there's a reason why many of the Senate's rules have barely changed since the late 1780s. Lawmakers can filibuster the rules change. In other words, they can block moving the issue to floor by objecting. And Reid has to marshal a super-duper majority, in this case, 67, to clear the filibuster and move on to the final passage.

But, there are ways around that, too. There is a special province for the leader to switch the cloture rule on the first day of the new Congress early next year without facing a filibuster. It's hard to muster 60 or 67 votes. But 51 is doable. And that's why Republicans fear the "nuclear option," as they've dubbed it.

"The filibuster looks totally different depending on whether you are in the majority or the minority," said Senate Historian Don Ritchie.

Republicans are upset now. But the shoe was on the other foot back in 2005 when Frist came close to throwing the switch on the nuclear option back when the Democrats were in the minority.

A new senator took the floor and argued against what Frist hoped to do.

"Everyone in this town knows if the majority chooses to end the filibuster, if they choose to change the rules and put an end to democratic debate, then the fighting and the bitterness and the gridlock will only get worse," the senator said, then a member of the minority party.

That senator was a freshman from Illinois named Barack Obama.

So Reid's finger is near the nuclear button. Will he push it? No one knows. But perhaps a study of the Cold War can serve as a guide. The United States and Soviet Union subscribed for years to the principle of "mutually assured destruction." That meant both sides faced certain devastation if they went to war.

But, many Cold War academics often point out that the doctrine of mutually-assured destruction is a deterrent. Reciprocal destruction creates an equilibrium - keeping the sides from going to war.

And such a scenario of mutually-assured destruction could shield the Senate from a nuclear winter.