Who says the vice president has no power?
Before Wednesday, the president had been walking a careful line on same-sex marriage: He officially opposed it, but professed to be "evolving" on the issue. The "evolving" language allowed the president to have it both ways: He could maintain his opposition to keep from alienating socially-conservative swing voters while also offering an implicit assurance to gay donors and the Democratic base that he is actually on their side.
Biden's comments made that stance untenable. And it wasn't just what Biden said - it was the immediate attempts at damage control by the White House and Obama campaign, which dubiously claimed that the vice president's comments did not go beyond the president's position. A source in the fundraising community told CBS News that the damage control effort seriously angered the gay donors and bundlers the president is counting on to fund his super PAC and campaign. Suddenly, the wink and nod represented by the "evolving" claims wasn't good enough, with donors and bundlers vowing to stop raising and giving money until the president did what they considered the right thing.
It wasn't just about the donors. Biden's statement shifted the spotlight to social issues and away from the economy, the issue that voters overwhelmingly cite as their most important concern. The president did not want to have to spend the next six months answering questions from reporters and making headlines about same-sex marriage while Mitt Romney relentlessly hammered him on the state of the economy. Better to get it out of the way now, when many voters have yet to fully tune in, than have to play defense straight through until November.
It's possible that Mr. Obama would have made his comments without Biden's push: A senior administration official told CBS News that the administration had been looking for the right time to make the announcement, and that Biden simply gave them the opportunity they were waiting for. Maybe. But the immediate reaction to Biden's comments suggested that Obama's camp was relatively happy with the status quo and was not looking to take on the issue.
What impact will the announcement have on Mr. Obama's re-election chances? It's hard to say. In one sense, the answer is not much: There aren't many voters out there for whom same-sex marriage is the most important issue. And strong supporters of same-sex marriage, who tend to be Democrats, were not going to back Romney - who supports of a federal constitutional amendment to define marriage as the union of one man and one woman -simply because of Mr. Obama's wishy-washy stance. Strong opponents of same-sex marriage, meanwhile, were largely going to vote against Mr. Obama no matter where he stood on the issue.
But that doesn't mean it doesn't matter. The statement will likely lead to more donations to Mr. Obama's campaign from gay and socially progressive donors, which can now pour money into the presidential coffers without reservations. It could provide motivation to social conservatives whose enthusiasm over the Republican nominee has thus far been muted at best. It's hard to guess the impact on swing voters in a state like Ohio, where the contest could come down to a few thousand votes - though a case could certainly be made that the announcement will alienate more voters than it will win over.
It's worth noting that Mr. Obama's announcement left him some wiggle room: He said that his support for same-sex marriage was a personal opinion, and that he still believed that the issue should be left to the states to decide. That frees him from having to take any sort of concrete action to back up his words, and it means that he can claim to voters in a state of North Carolina - where voters on Tuesday overwhelmingly passed an amendment to constitutionally ban same-sex marriage - that he isn't forcing his position on them. It also means that for all the talk that Mr. Obama has finally taken a stand, he still hasn't gone as far as the strongest same-sex marriage supporters would have liked him to.
Still, Mr. Obama's announcement was undeniably historic. And, hard as it may be to believe, it may not have come completely down to politics. It's possible that the president simply decided that Biden's comments offered a clear opportunity to put himself on what he sees as the right side of history - and that doing so was worth the possible fallout. In discussing the decision, Jonathan Rauch of the Brookings Institution unearthed a Mark Twain quote beloved by Harry Truman: "When in doubt, do right. This will gratify some people and astonish the rest."
With reporting by Norah O'Donnell and John Dickerson.