Today is a pretty big day in the world of U.S. environmental policy.  The D.C. Circuit Court of Appeals will hear oral argument on state challenges to the U.S. Environmental Protection Agency’s “Clean Power Plan.”  The Plan, promulgated under existing Clean Air Act authority, represents one of the centerpieces of the Obama Administration’s efforts to combat climate change.  It also underlies the U.S. commitment to the Paris climate agreement signed in December.

So the stakes are high.  Folks were lining up early to get a spot in the courtroom today, as Denise Grab of NYU Law’s Policy Institute tweeted at 5:30am:

75+ people already in line at 5:30 am in the rain for #CleanPowerPlan oral argument. That’s dedication. pic.twitter.com/gvblZqAefp

— Denise Grab (@denisegrab) September 27, 2016

The court debate will hinge on whether or not the EPA is limited to only regulating sources like power plants “behind the fence line” — in other words, only requiring on-site emissions reductions technologies — or whether the EPA can require grid-wide emissions reduction policies, like cap-and-trade or energy efficiency programs.

My UCLA Law colleagues Ann Carlson and Cara Horowitz, along with William Boyd (University of Colorado Law), describe the basic argument here on Legal Planet:

The Clean Power Plan uses the grid’s interconnectedness to reduce power-sector emissions in an efficient, effective way. The Plan would cut carbon dioxide emissions significantly by 2030 – to about a third below 2005 levels. The rule justifies that level of reduction by calculating, among other things, the potential for shifting generation toward low- and zero-emitting sources and away from coal-fired power plants. Yet the coal industry and conservative attorneys general who are challenging the CPP claim that we should ignore the interconnected electricity machine and treat its component parts – power plants – separately.

The case magnifies the intensity of the coming presidential election.  Regardless of the outcome today, the case will likely be appealed to the U.S. Supreme Court.  Due to a Republican senate blockade on Merrick Garland, President Obama’s pick to fill the open seat from Antonin Scalia’s death last February, the court only has eight justices.  A 4-4 tie on this appeal will let the circuit court opinion stand.

So whichever candidate is elected president and fills that seat (or causes Republicans to buckle and confirm Garland in the lame duck session) will have a major impact on the national and international climate fight.

And as last night’s presidential debate showed, the candidates diverge sharply on this issue.  Hillary Clinton was the only candidate to mention clean energy jobs and attack Donald Trump for his past statements on climate change as a hoax by the Chinese to gain a competitive advantage.  He denied making that claim, but his 2012 tweet says otherwise.  Meanwhile, his campaign manager affirmed today that he doesn’t believe in the science that humans are causing climate change.

So the choice could not be clearer on this issue in November.  And the court decision stemming from oral argument today will loom large, regardless of how much the media pays attention to climate change during this campaign.