If 90% of the public supports something, it's either an obviously good thing or a hideously terrible thing that everyone has been duped into supporting. In the case of the oft-parroted 90% support for background checks, it's a mixture of the two and some other things as well.
The Quinnipiac poll where the 90% figure comes from is as follows:
Do you support or oppose requiring background checks for all gun buyers?If 91% of people supported universal background checks, and that's all that was in the Toomey-Manchin amendment, it would have passed overwhelmingly. There are three things that kept it from doing so: a large number of that 91% support the idea of universal background checks without realizing the implications, Toomey-Manchin had a lot of other problems, and many of the people voting on it are up for reelection next year.
Don't know: 1%
If actually 91% of people support something, and you're worried about your next election, you would vote for that thing, not against it. "But it's Republican primary voters!" Nope. "But it's Democrats running in red states!" Nope. The Quinnipiac poll shows 88% Republican support, 90% Southern support, and 91% Western support for it as well. Again. If these numbers are right and are actually consequential.
Three of the four Democrats who voted against Toomey-Manchin are up for reelection next year in red states. But even if these states were 100% Republican, 88% of them said they support universal background checks. The fourth Democrat, Heidi Heitkamp, was just elected last year, so she has another 5 years before she has to worry about reelection. Her statement on her vote includes:
"I’ve thought long and hard about this, I’ve taken the tough meetings, and I’ve heard overwhelmingly from the people of North Dakota; and at the end of the day my duty is to listen to and represent the people of North Dakota."
But the people of North Dakota presumably also support it in the high 80s or 90s! Right? RIGHT???
So what's the problem then? If so many people support an idea, why don't their representatives? It must be "willful lies" of the evil "gun lobby"! Nope. It's just that no one really cares. While a lot of people might like the idea of gun control, only 4% of people think it's the most important problem we face. Just as many people think North Korea and immigration are our worst problems, and more people think moral decline is. So while it may have popular support in an isolated poll, it's not going to bring people out to the only poll that matters - their local precinct next November. That means Senators and Representatives are free to use their heads when considering issues instead of poll numbers. So that means they were able to see the other problems with the bill and rightfully oppose it.
There are a few problems with the idea of universal background checks itself, of course. First, it's great theoretically for everyone who buys a gun to have a background check, but it is logistically difficult. Background checks are already mandatory for licensed gun dealers, the only people setup to conduct them. Extending that requirement to everyone in the country would in effect end legal private gun sales. If I had a gun I wanted to sell to my neighbor, how would I go about getting a background check on him? The background checks mandated by federal law aren't as simple as those used by tenant or employee reporting agencies. Perhaps a system could be setup to streamline the process, but Toomey-Manchin didn't do that, otherwise it might have made their case easier to consider. Technically, Congress shouldn't have the power to regulate this kind of commerce anyway, since it' may not occur over state lines.
Second, it doesn't count gifts, donations, or trades. Adam Lanza, the Sandy Hook murderer, stole his weapons from his mother, who passed a background check. Even banning gifts and trades wouldn't prevent people from stealing guns. For those new to the planet, criminals don't follow the law.
Third, many, if not most, murderers wouldn't be stopped by a background check anyway. Adam Lanza might have been an oddball, but he didn't have a diagnosed mental illness or criminal background. He could have gone to the nearest gun store, bought an AR-15 and two pistols, and done the same thing, even after Toomey-Manchin or even harsher gun control measures were in effect.
So yes, it's easy to use isolated poll numbers and grieving families to bludgeon people who stop your agenda dead in its tracks, but occasionally there are actual ideological and substantive reasons for them to do so, even if they are politicians!