At the United Nations today, President Obama gave a decent speech about climate change. He hit a number of key points: Climate change is not just an “urgent and growing threat”; it’s the most important and consequential issue of the 21st Century. The science is undeniable, as are the signs of rising danger all around us. We know what we have to do to combat that threat. We are dangerously close to condemning the next generation to a future that “is beyond our capacity to repair.”

But the most important part of the speech may have been the implicit acknowledgment of the limitations that are likely to continue to constrain action for the foreseeable future. He said:

“Today, I am here personally, as the leader of the world’s largest economy and its second largest emitter, to say we have begun to do something about it….but let me be honest, none of this is without controversy. In each of our countries, there will be interests that will be resistant to action. And in each country, there is a suspicion that if we act and other countries don’t, that we will be at an economic disadvantage. But we have to lead…

“Yes, this is hard. But there should be no question that the United States of America is stepping up to the plate. We recognize our role in creating this problem. We embrace our responsibility to combat it. We will do our part. And we will help developing nations do theirs. But we can only succeed in combating climate change if we are joined in this effort by every nation, developed and developing alike. Nobody gets a pass.”

Obama outlined policy steps to tie climate measures to international aid and development, which Philip Bump summarizes here. He recapped steps the administration has already taken, such as outlining tougher fuel efficiency standards, and ones that are underway, such as the cuts to American emissions the EPA is currently drawing up.

But broadly speaking, if solving this problem turns heavily on “embracing our responsibility to combat it,” it’s just a reality that Congress is going to remain a major obstacle, probably for many years. Those are among the “interests that will be resistant to action” that Obama referenced, in a moment of understatement.

The prospects of ambitious cooperative action from the coming United Nations climate talks — which Obama called for — are impaired by the familiar divide between rich and poor nations, in which the former talk a good game but do little while the latter are the ones who bear the brunt of ongoing failure. One key thing Obama can do on this front will come later, when the United States proposes its own carbon reduction targets as part of these talks.

“Putting out an ambitious goal is really important and builds on the major steps the Obama administration is already taking to cut carbon pollution,” Tiernan Sittenfeld, a vice president at the League of Conservation Voters, tells me. “What we’re willing to do sets an example for other countries.”

And yet, because any international climate treaty requires a two-thirds majority of the Senate, the administration is reduced to exploring ways of pursuing a treaty that isn’t legally binding and wouldn’t require Senate ratification.

Meanwhile, it’s looking increasingly like the best case scenario for Dems this fall is a 50-50 Senate with Joe Biden as tiebreaker, and the odds of a GOP takeover are perhaps somewhat better than even. Mitch McConnell has vowed that a GOP Senate would use the budget process to chip away at Obama’s environmental regulations. While Obama would veto such efforts, he might be tempted to compromise here and there. As David Leonhardt puts it: “Republican control of the Senate would probably mean less climate regulation — and more carbon emissions.”

Environmentalists have worked hard to prove that climate can matter in electoral politics, but the fact that the bright spots are so rare — the Michigan Senate race may be the only real example of success — doesn’t exactly bode well. Yes, Democrats will very likely take back the Senate in 2016 if they lose it this year. And yes, the People’s Climate March attended by over 300,000 people suggests a real movement is in the making. But for a variety of reasons control of the Upper Chamber will probably be unstable and closely contested, with very narrow majorities in either direction, for years to come, no matter what happens with this movement.

So a great deal turns on Obama’s implementation of the new EPA rules and on the outcome of these international climate talks. Yet Congress can frustrate progress even on those fronts.