Ben Nelson apparently signed on to the trigger idea today. I agree with Ambinder (and disagree with the AP story, which buried it in the 10th paragraph) that this is potentially very important news. Very important. Nelson is pretty clearly the most conservative Democrat. Olympia Snowe, by all accounts, loves the trigger; at any rate she would almost certainly vote for it. If they are the 60th and 61st vote (or 59th and 60th until the Massachusetts vacancy is filled), then it seems very, very likely that a bill with a public option trigger could get through the Senate. Not certain -- Kent Conrad, for one, needs to be budged from co-ops to public option w/trigger, although I suppose that they could write co-ops into the bill in addition to a triggered public option. But if Nelson climbs on board, it sure looks to me like the bill is in very good shape. Don't forget that Snowe could very well bring up to three Republicans with her, too.
The nice thing about a trigger is that it's adaptable for compromise. Easy to imagine: the Senate passes a weak trigger (that is, a trigger this is difficult to activate, and would only establish an insignificant public option if activated). The House passes a plan with a public option. That's a pretty easy deal to cut, no? Keep the robust public option from the House, make it triggered for the Senate, and make the trigger somewhat easier to reach. If marginal votes in the Senate supported the original idea, it's hard for me to believe that they would be so strongly against that compromise that they would filibuster it. And while liberals in the House (and out of it) aren't going to be thrilled with only a triggered public option, it remains as true now as ever that they really have nowhere to go -- and I think liberal Members of the House could make the case to their supporters that they got a pretty good deal.
Of course, the deal isn't a deal until everyone signs on to it, and I've only read the reports about what Nelson said, so I don't know what other qualifications he put on his potential support, but it may be that Snowe (and those she's working with) have found the formula to get this done.