I get the feeling that since Democratic senators know how many "No" votes the deal can withstand, they're all drawing straws as to who actually gets to vote against the deal while not blowing it up.
Not drawing straws, trading horses.
Pelosi's work in the House should be educative here. Notice how many times she passed bills with the bare minimum majority necessary to get them through the House -- when the numbers cut that close, you can be confident that the Leader is giving each Democratic no explicit permission to take their principled vote, in exchange for getting their yes later on when somebody else has to dissent. Similarly, Boehner's flailing displays what happens when a party leader really doesn't think they know who's going to vote no -- rather than let the bill fail, they pull it before the vote.
Believe me, no Democrat is voting no on any Democratic bill without permission from the bosses, and they're doling out those nos carefully. Schumer may end up voting no (in fact, I kind of expect him to), but Reid's giving him the loose rein. As long as the bill passes, who cares?
At this point, I think the previous vote on the Iran deal should really come into focus, because the purpose of it was explicitly to buy breathing room by changing killing the deal from a negative action (that happens unless Congress acts) to a positive action (that requires both houses to override a Presidential veto). All the Democrats need to provide is 145 representatives in the House (they have 188) OR 34 Senators (they have 46). They don't even need to sustain the veto in both houses. The chance of the deal failing is basically zero.