The main problem with the article is that the author attempts to draw a lot of conclusions from only one source: a Pew Research Poll. For one thing, the conclusions that the author tries to draw are pretty much directly contradicted by another poll from Gallup:
http://www.gallup.com/poll/186284/despite-criticism-nra-enjoys-majority-support.aspx
In this poll, the NRA is viewed favorably not only on the whole, but also among self-identified moderates and, albeit by a slight margin, among non-gun owners.
Also, the author makes the mistake of assuming that just because a particular group is in the minority (in this case whites) that said group will be on the losing side of a political issue. African Americans are and always have been a minority in the U.S., yet they are still a very powerful and influential group in American politics. Likewise, the LGBT community has enjoyed several political victories in the past few years despite comprising only 3.8% of the population.
Finally, he's assuming that total for/against support translates directly to voters at a ballot box, but it doesn't. This is why you often see certain polling agencies conduct polls of
likely voters as opposed to simply registered voters. Simply put, a very large percentage of the population (>40%) doesn't even vote at all and a larger percentage still probably only turns out for the Presidential elections every four years. Also, he's assuming that everyone who says that they support gun control feels so strongly about it that they will always vote for whichever candidate supports gun control. There are lots of issues that a given person might consider when deciding who he/she is going to vote for and gun control isn't always the most important one.
Also, people who oppose gun control tend to feel more strongly about it than those who support gun control according to this Pew poll from 2013:
http://www.people-press.org/2013/05/23/broad-support-for-renewed-background-checks-bill-skepticism-about-its-chances/
In particular, by a 41-31% margin, gun control opponents are more likely to never vote for someone who disagrees with them on the issue than gun control supporters. Likewise, gun control opponents are more likely to contribute money to an organization which supports their position and to contact elected officials.