At a caucus even in Iowa today, Hillary Clinton finally came out in opposition to the Keystone XL pipeline, which would carry tar sands oil from Canada down to the Gulf Coast.

Clinton's shift is a testimony to the extraordinary work done by the climate movement since the defeat of the cap-and-trade bill in 2010. Using Keystone XL as a linchpin issue, the movement expanded, putting thousands of marchers in the streets, pulling a few billionaires onto their side, and generating over $2.6 trillion in fossil-fuel divestment.

Most of all, activists helped to intensify the feelings of left-leaning voters on climate generally and Keystone specifically, creating a political landscape in which a national Democratic candidate simply can't afford to be on the wrong side of the issue.

Clinton has been dodging this issue for years

Climate activists have been pressuring Clinton for years to take a stand on the pipeline. Until now, she has dodged the issue, citing her work as Secretary of State, clearly hoping to run out the clock until the administration made some kind of final decision.

But with Obama's decision-making process seeming to drag on forever, she evidently decided that the risks of waiting (and offending her base) outweighed the risks of getting out ahead of Obama. Here's what she said today:

I was in a unique position as Secretary of State at the start of this process and not wanting to interfere with ongoing decisionmaking that the President and Secretary Kerry have to do in order to make whatever final decisions they need. So I thought this would be decided by now, and therefore I could tell you whether I agree or disagree. But it hasn't been decided and I feel now I've got a responsibility to you and to voters who ask me about this. I think it is imperative that we look at the Keystone pipeline as what I believe it is, a distraction from the important work we have to do on climate change. And unfortunately, from my perspective, one that interferes with our ability to move forward with all the other issues. Therefore, I oppose it.

Naturally, Clinton's competitors for the Democratic nomination took the opportunity to remind us that they were way ahead of her. Sen. Bernie Sanders (I-VT) stressed that he had "vigorously opposed the Keystone pipeline from the beginning." O'Malley faulted Clinton for dragging her feet on issues from gay marriage to climate change.

Clinton's new stance might not matter very much

Despite the iconic role Keystone has taken in energy debates, it's not clear that Clinton's statement of opposition is going to have any significant effects, or that it carries any particular political risks.

Obama will make whatever decision he was going to make. Clinton's opposition will endear to her base, which is helpful in a primary, but it's not like any greens were thinking of defecting to the GOP. It will outrage Republicans, but when are they not outraged?

Could it hurt her in the general? Probably not. Public opinion is narrowly on the side of the pipeline, but that opinion is shallow and sloshy. Most people don't know much about it and don't have much of an opinion; those who do have opinions will only have them reinforced by Clinton's decision. Whoever Clinton's general election opponent turns out to be will inevitably attempt to use Keystone as evidence of the Democrats' "war on energy," but that too was inevitable, no matter what Clinton said.

In short, this is a great symbolic victory for climate change campaigners and a clear demonstration of their growing political power, but in terms of larger political dynamics, it probably won't change much.