Big news for the Senate today: In a vote that’s still open (as I write this) so that Heidi Heitkamp (D-N.D.) can fly back and cast the vote to get to 60, the Senate is going to invoke cloture and defeat a filibuster against Bureau of Alcohol, Tobacco, Firearms and Explosives director nominee B. Todd Jones, ending years of acting directors at that agency.

As I said earlier this week, this one is a key test of whether the Senate deal will hold beyond the initial seven agreed-upon nominees. It was touch-and-go; only five Republicans initially voted for cloture, with Lisa Murkowski eventually flipping her vote and joining them after what appeared to be extensive efforts to change her mind. It made for dramatic CSPAN-2 watching, at least until the long wait for the flight from North Dakota began.

So the deal holds. Still unknown, however, are the answers to several questions:

• What would, under the deal, trigger a nuclear reaction? A single successful filibuster of any executive branch choice? A certain (low?) number? Certain types of successful filibusters — say, “nullification” filibusters designed to keep an agency from running?

• Republicans are handling this by tag-teaming their votes; in five close cloture votes so far under the deal, only John McCain, Susan Collins and Murkowski have voted yes every time, while another dozen or so have taken turns filling in the rest of six needed. Clearly Murkowski has not committed to always voting for cloture, either. So how is this organized? Are there six (or more) who have agreed to supply votes if needed? Or has McCain (or Collins, or some combination) agreed to find six?

• Are Republicans responsible for exactly six, no more? Today’s evidence suggests that’s the case; no seventh vote could be found so that Heitkamp could stay home. Presumably, the same would be true if any Democrats vote against cloture — which gives marginal Democrats quite a bit of leverage on any close votes.

As I’ve been arguing, everyone would be better off if the Senate simply switched to simple majority cloture, which could be done under the regular rules if all the Republicans who have joined the tag-team at least once combined with all the Democrats who were prepared to go nuclear. Without that, the Senate deal is a lot less stable than it could be.

However, for now, it does seem to be holding. And the principle that it takes a supermajority to change the rules of the Senate also holds. It’s still suboptimal, but at least the Senate is getting its work done.