Barring an unforeseen foreign policy crisis of some kind, Monday June 2 is overwhelmingly likely to go down in history as the single most important day of Barack Obama's second term in office. It's the day on which the administration will unveil its long-in-the-works Clean Air Act regulations of existing power plants. Those rules — reportedly planning to cut emissions by 30 percent relative to a 2005 baseline — will be the most important climate change measure Obama takes during his eight years in office, and as such they'll be the most significant thing he does in his second term.

Once upon a time, of course, the White House had a fairly ambitious second term agenda.

there's no sign that Obama's reelection succeeded in breaking the fever and denting Republican resolve to block his proposals

Indeed, the "fiscal cliff" deal with congressional Republicans that raised less revenue than progressives wanted and less than it seemed like Obama could get by playing hardball was justified in part in the name of that agenda. The theory was that the president needed to clear the decks of the tax issue in order to make room for action on comprehensive immigration reform, gun safety regulation, a new surface transportation bill, and maybe even an overhaul of K-12 education.

But a fairly weak gun bill hashed out by Senators Joe Manchin (D-WV) and Pat Toomey (R-PA) was filibustered to death in the Senate. An immigration bill that did manage to pass the Senate is languishing in the House where John Boehner won't give it a vote.

In other words, there's no sign that Obama's reelection succeeded in breaking the fever and denting Republican resolve to use the tools at their disposal to block the president's proposals. And at this point, nobody thinks Democrats are going to win a House majority in the 2014 midterms. The only real question is will they lose a few Senate seats or will they lose enough Senate seats to make Mitch McConnell the majority leader.

So we're left with executive action. And while EPA regulation, tellingly, tends not to be heavily featured on the administration's list of We Can't Wait unilateral uses of presidential authority, it's much more substantively important than things like raising the minimum wage for federal contractors.

It's important not only because climate change is an issue of enormous significance, but because the text of the Clean Air Act really does give the executive branch power that matters. The White House can't unilaterally change the wage structure of the United States or create a path to citizenship for unauthorized immigrants, but it really can revolutionize the environmental practices of the electricity sector — a sector that, as seen below, is responsible for about 38.4 percent of America's total carbon dioxide emissions.

Combine that with the strict new fuel efficiency rules for the transportation sector that the administration put into place at the end of the first term, and Obama has the chance to make a major dent in American climate emissions despite full-tilt congressional opposition.

energy issues are highly regionalized, and the risk of an anti-Obama backlash from Democrats representing coal-dependent areas is real

Of course, that depends on how strict a rule is adopted. The official policy hasn't been announced yet, but Sunday's reporting suggests they will propose a 30 percent cut relative to where they were in 2005. That's somewhat less dramatic than it sounds, as power-plant emissions today are already about 10 percent below that level.

The stakes here are high.

Adopt a rule that's too lenient, and the president will miss a unique opportunity to make an impact on one of the most important issues facing the world today. Adopt a rule that's too strict and he'll risk a congressional backlash that could ultimately undermine the EPA's ability to do anything. Republicans are sure to cry "overreach" no matter where the administration comes down, but they would need Democratic defectors to really change anything. Yet energy issues are highly regionalized, and the risk of an anti-Obama backlash from Democrats representing coal-dependent areas is real. Striking the right balance will be tough, and regardless of what the rule looks like it will be some time before we know whether Obama's done it.

But either way, make no mistake this — not the Keystone XL pipeline or congressional Benghazi hearings or clearly doomed transportation plans — is the political story to watch of our time.