Harry Reid is on the Senate floor this morning complaining about Republicans' frequent use of the filibuster to block President Obama's executive-branch appointments and judicial nominations. “There are currently 75 executive branch nominations ready to be confirmed by the Senate," he said, "waiting an average of 140 days."

By many accounts, Reid is poised to make drastic changes to the filibuster today — so that most judicial nominees would only need 51 votes for confirmation. (Supreme Court nominees could still be blocked by filibuster.) This is the so-called "nuclear option."

But how would this actually work? Usually it's simpler for the Senate to change its rules at the beginning of a session, but there are other options for modifying the filibuster right now — they're just unprecedented:

As the University of Miami's Greg Koger explains here, there are a number of ways the Senate can change its rules mid-session.

The most prominent, detailed in this CRS report, is for the Senate to declare the filibuster unconstitutional. One way for this to happen would be for the presiding officer — most likely Vice President Biden or Senate president pro tempore Pat Leahy — to declare it unconstitutional, either in whole or in part (for example, for just judicial and executive nomination). That's a break with Senate precedents, by which the body as a whole has to decide constitutional issues. A Republican Senator would likely appeal, and then a Democratic Senator would move to table the appeal, which would then go to a majority vote of the chamber. If a majority tables the appeal, the filibuster is effectively amended or ended.

Alternately, a Democratic Senator could submit a point of order that the filibuster is unconstitutional (in whole or in part), after which the presiding officer would submit the question to the Senate as a whole. Usually, such a question would be subject to debate, but the presiding officer could break precedent and declare it undebatable. A Republican Senator would likely appeal, and then another Democratic Senator would motion to table that appeal, which would go to an immediate vote. If a majority votes to table, the Senate would proceed to vote on the original motion to declare the filibuster either wholly or partially unconstitutional. If a majority votes yes, the filibuster is amended or ended.

But Koger notes there are ways to change the rules without making any declarations about constitutionality. For example, the Senate could set a precedent whereby motions to suspend the rules are nondebatable, and only require a majority vote. That way, whenever there's a threat of a filibuster, the majority could just move to suspend the rules and proceed to a final vote, a move which would succeed as long as a simple majority is in favor. Boom, filibuster ended.

Now, it's definitely more straightforward to change the rules on the first day, as the Senate can simply decide that the previous Congress's rules are not yet in effect and move to adopt new ones by a simple majority vote. But there's nothing stopping Harry Reid from changing the rules whenever if he's got 51 votes with which to do it.

Update: Senate Democrats just voted to get rid of the filibuster for executive-branch nominees and judicial nominations. (Legislation and Supreme Court nominees can still be filibustered.)