There was a moment on the Senate floor on Monday when Sen. Harry Reid could've done pretty much anything he wanted. There was nothing the Republicans could've done to stop him. And he knew it. He just didn't take advantage of it.

"This afternoon, I offered a consent agreement dealing with the budget," he said. "I withdrew that because we didn't have anyone here to object, and I had an inkling that there would be an objection if a Republican were here." At that point, a Republican was there. Sen. Ted Cruz, to be exact. And he objected.

Reid's restraint is a good illustration of how much the Senate runs on norms rather than rules. There's nothing in the rules stopping Reid, or anyone else, from taking advantage of an empty floor. They just don't take advantage of empty floors. Partly that's because it's a crummy thing to do. But partly it's simple self-preservation. As one Senate aide put it to me, Republicans "could repeal [Obamacare] by the same method."

In his book "Filibustering," Gregory Koger tells the story of senator Thomas Gore, a blind Democrat from Oklahoma. Gore was working with two colleagues to block a banking bill. After taking a long shift on the floor, Gore threw the filibuster to his ally, Sen. William Stone. But Stone had stepped out for a moment, and Gore, being blind, hadn't notice. The majority did, however, and they took the opportunity to end Gore's filibuster. A bit low? Sure. But that was the norm back then. Filibusters were wars of procedural attrition. Gore's filibuster never had the votes to kill the bill. So the only question was how and when the majority would manage to maneuver around his obstruction.

The Senate runs on norms even more than it runs on rules. There's much that senators simply didn't used to do because, on the one hand, doing so would be crummy, and on the other, doing so would be destructive. Routinely filibuster everything, for instance. Routinely block the other side from offering amendments. Routinely offer endless non-germane amendments. Routinely use budget reconciliation for matters that really aren't about the budget. Routinely use secret holds.

But that's the problem with using norms rather than rules. Once they're broken, they're broken forever. We've broken so many norms in recent years that the Senate of today bears little resemblance to the Senate of 1983, much less the Senate of 1953. But because there was never a formal fight over a rule change, and because the changes came gradually, we didn't notice. Now it's too late. The norms that once protected the Senate are largely gone. And we haven't erected new rules in their place.