Tomorrow, the entire House Democratic caucus will vote to decide the ultimate winner.

A great deal hinges on the outcome. Dingell, the longest-serving member of Congress, has close ties to Detroit’s automakers. For decades he’s led efforts to thwart the plans of some other Democrats to limit tailpipe emissions and raise fuel-efficiency standards. Waxman, just elected to his 18th term, has long advocated for these changes. With an Obama administration set to take the White House in January, environmentalists are drooling at the thought that the liberal Californian could be the gatekeeper to many of Obama’s ambitious green-energy plans — which would have to pass through the Energy and Commerce Committee.

There are no forgone conclusions. Dingell is monolith with enormous sway over his colleagues. But Waxman, as today’s vote proves, is no slacker when it comes to whipping support. Indeed, he’s won contests against seniority before.