Darren Samuelsohn of ClimateWire has a very helpful piece re-assessing the prospects for cap-and-trade legislation in the Senate. A carbon cap can't go into the budget reconciliation bill this year, which means Republicans can filibuster, which means it'll need 60 votes to pass. But climate legislation isn't doomed yet. A bevy of swing Democrats, from Mary Landrieu to Carl Levin, still sound sanguine, as they watch the debate over the House energy and climate bill slowly unfurl. One crucial dynamic: If Waxman and Markey can drag along key conservative Dems in the House, that could help garner votes in the Senate, too:

Consider Rep. Mike Ross (D-Ark.), a member of the fiscally conservative Democratic Blue Dog Coalition. Ross also sits on the Energy and Commerce Committee, and his vote could be pivotal to the climate bill's chances for success both in the House and with Arkansas' senators, Pryor and Blanche Lincoln (D).

"I guarantee you if Mike Ross is OK with it, it goes a long way with me if he's with it," Pryor said. "I still have to make a judgement myself. But if he's OK with it, it means a lot to me."

Landrieu said she too would be mindful of how the 51 members of the Blue Dog coalition vote on the climate and energy bill. "It will help a lot if Blue Dog Democrats can come to terms, but it's still important to try to reach as much bipartisan consensus as possible," she said.

So a great deal depends on Waxman's much-praised legislative skills. And what about Republican "yes" votes in the Senate? Well, Susan Collins of Maine has said she's in favor of cap and trade. Olympia Snowe once introduced a bill on the issue. The George Voinovich, who's set to retire next year, is sour on the odds of crafting policy that can placate both green groups and 60 senators, but he's still hoping to reach a consensus, and told ClimateWire that senators are quite mindful that the EPA will soon have the authority to regulate carbon on its own if they don't act.

Meanwhile, there's the always-mercurial John McCain, who has sponsored cap-and-trade bills in the past, but at this point claims he hasn't been consulted by either the White House or Senate Democrats. I've been trying find out if there's any back story hereā€”in theory, one would imagine he could be a very strong ally for Obama on this issue. With any luck, more on that later.

There's also some tension between the House and Senate legislative approaches. The Waxman-Markey bill is a gigantic omnibus measure straddling everything from efficiency grants and electric-grid upgrades to carbon caps. This is the all-or-nothing approach. The Senate, meanwhile, is doing things piecemeal. Jeff Bingaman's energy committee is slowly marking up its own energy legislation, which will include renewable-power requirements and appliance standards and green-job training but no carbon cap. (The cap is being put together by Barbara Boxer's environmental committee.) How this all gets wrangled together into law is still an open question.

--Bradford Plumer