Reid has threatened to change the rules. ( Kevin Lamarque/REUTERS) Reid has threatened to change the rules. (Kevin Lamarque/REUTERS)

It looks like Senate Democrats and Republicans have reached a tentative deal to avoid a showdown over the filibuster and confirm several of President Obama's executive branch nominees. But if we do get back to the so-called "nuclear option," there's still confusion over what Senate Majority Leader Harry Reid (D-Nev.) has been planning and how -- or whether -- it would even work. Since the threat is likely to stay on the table, it's worth explaining what would happen. 

In general, changes to the rules can be filibustered, and a 67-vote threshold is required to cut off debate on any such change. But a ruling from the presiding officer of the Senate on the rules can be affirmed or overturned by a majority vote.

We don't know exactly how all this would go down. Democrats don't want Republicans to know their plans, because that would make it easier to come up with procedural ways to gum up the process.

A likely scenario: Reid invokes cloture on a nomination. If Republicans object, Reid raises a point of order and asks the presiding officer for a ruling from the chair on how many votes are needed to confirm an executive branch nominee. The presiding officer turns to the parliamentarian and ask for a ruling. The parliamentarian responds that according to precedent, 60 votes are required. The presiding officer then declares filibusters on executive branch nominees invalid, followed by a vote requiring 51 votes to uphold the ruling.

What we don't know is what kind of point of order Reid would raise. He could take the path Republicans nearly took in 2005 with judicial nominees, in which the point of order was based on the Senate's "advise and consent" power for such nominations. Or there's the path reformers took in 1975, which rested on the constitutional right to change the rules.

There's also the 2011 example, when Democrats voted to change the the rules on non-germane amendments without invoking the Constitution. That change hinged on interpretation of what is and isn't "dilatory" rather than constitutional issues. But to make a change this dramatic, "my hunch is that he's going to need some constitutional basis for the point of order," congressional expert Sarah Binder said.

The big issue: According to precedent, the presiding officer is not supposed to make constitutional rulings. Those rulings are supposed to be turned over to the Senate for a vote. Should that happen, there would be filibusters along the way and a change would be out of reach.

"It is not clear that any such form of proceeding can be proposed that would not require violations of existing rules in the process of changing them," a February Congressional Research Service report on the issue says.

There has been some talk that there's more constitutional leeway for the rules to be changed on the opening day of a new Congress. That's what Sens. Jeff Merkley (D-Ore.) and Tom Udall (D-N.M.) were pushing last year, and it echoes 1975 reform. While it might seem less threatening, there's no constitutional reason why rules changes would need to occur at the beginning of the session, experts say.

Reid has accurately noted that the Senate rules have been changed 18 times in the past 36 years. Merkley has prepared a memo detailing those changes:

Merkley Memo On Filibuster

But none of those changes involved the number of votes required for cloture.

"The process is conceptually the same... but it's different in the target," Binder said.

"I have actually seen it done," former Senate parliamentarian Robert Dove told MSNBC Monday. That was the 1975 case. After years of attempts at reform in the face of filibusters of civil rights legislation, a bipartisan group got a majority vote agreeing on a 50-vote threshold to cut off debate on rule changes at the beginning of a Congress.

But the change opened a can of procedural worms, leading to two weeks of conflict over the rules. Opponents sat down to negotiate, and ultimately the change was undone by unanimous consent. Reformers got the change they had really wanted -- a three-fifths rather than two-thirds threshold to cut off filibusters. (For a very detailed rundown of that fight, see this paper.)

In the end, the threat of the rules change was enough to bring about a deal. That looks like the tactical precedent Reid is following today.