America’s Climate Security Act — aka the Lieberman-Warner bill — passed through its first markup hearing today, but not without losing support from the Senate’s most vigilant advocate for action against climate change, Bernie Sanders (I-Vt.)

The hearing was, in a sense, a tête-à-tête between Sanders and the bill’s primary author, deal-maker extraordinare Joe Lieberman (ID-Conn.). It was a chance for Sanders to attempt to improve the bill in ways he must have known would be rejected, and a way for Lieberman to do the actual rejecting — if only to keep his fragile coalition together.

All but one of Sanders’ proposed amendments failed badly, including bids to strengthen the auction of pollution allocations, lower the cap on emissions, earmark subsidies for renewable energies, demand accountability from the auto industry, and diminish industry’s capacity to stall simply by buying carbon offsets.

In most cases, the only man voting alongside Sanders to improve the bill was New Jersey Democrat Frank Lautenberg.

But by the end of the hearing it was clear: Lieberman had secured in advance all of the support he needed to move his bill to the full Senate Environment and Public Works Committee, and he wasn’t going to let Sanders spoil that success. In the end, with coauthor John Warner (R-Va.) — who was absent for health reasons — voting “aye” by proxy against the other Republicans, Lieberman didn’t need Sanders and led the effort to kill his proposals. At the close of the hearing, Sanders, protesting the weakness of the bill, voted no.

Moments after the vote, Brent Blackwelder, president of the environmental group Friends of the Earth, sent me this statement: “Senator Sanders is providing much-needed leadership. He has been the Senate’s strongest advocate for responding to global warming in a way that’s commensurate with what science says is needed. We plan to do all we can to make sure his important voice is heard.”

Now that it’s made it through the subcommittee, though, the Lieberman-Warner bill will face the tougher challenge of passing through the full committee.

With the exception of maybe Lamar Alexander of Tennessee — and that’s an extremely large maybe — it’s likely that John Warner will remain the only Republican “aye” when America’s Climate Security Act comes to a committee vote in weeks ahead.

So where does that leave things?

Well, Lieberman may have decided not to allow Bernie Sanders to play a constructive role in this process. But what he did do is bring Frank Lautenberg in on a big amendment package — announced at the beginning of the hearing as a “substitute amendment” — the details of which remain largely unknown. What is known is that Lautenberg voted to move America’s Climate Security Act forward today.

The question, then, is whether or not Lautenberg will make a bid for for more — whether his support is permanent or temporary. If he’s thrown his lot in with Lieberman and Warner, then the bill has a decent shot of making it to the full floor of the Senate.