Jonathan Bernstein makes a good point today about what I call "poll literalism," the idea that if a poll shows the public is on your side, that means the public is really on your side. In this case, the subject is background checks for gun buyers:
Sure, 90 percent of citizens, or registered voters, or whoever it is will answer in the affirmative if they're asked by a pollster about this policy. But that's not at all the same as "calling for change." It's more like...well, it is receiving a call. Not calling.
Those people who have been pushing for marriage equality? They were calling for change. And marching for it, demanding it, donating money to get it, running for office to achieve it and supporting candidates who would vote for it, filing lawsuits to make it legal. In many cases, they based their entire political identity around it.
Action works. "Public opinion" is barely real; most of the time, on most issues, change the wording of the question and you'll get entirely different answers. At best, "public opinion" as such is passive. And in politics, passive doesn't get results.
Public opinion is real. But it only matters if it's strong, and polls rarely measure that. It only matters if it determines who you're going to vote for (or against), and polls rarely measure that. It only matters if it means that lots of people are willing to make big hairy pains in the asses of themselves, and polls never measure that.
Public opinion is a start. You certainly need it, and politicians won't do much of anything without it. But it's nowhere near enough.