Democratic leaders are increasingly aware of that, particularly because many of their new members who are responsible for handing the party their majority are either strongly pro-gun or from states and districts where gun rights is a major issue.
Speaking from anecdotal experience, if there's one "right wing issue" that resonates with many lefties, it's gun rights.
If there's one thing that the article get wrong in a major way is that they fail to reference how many members the NRA has. I can't quickly find any direct links, but the NRA is one of the largest orgs by membership.
In addition to that, the membership is heavily weighted toward one-issue voters. I can't even send money to Republican organizations because I can't stomach the possibility that even one thin dime might go to an anti-gun Republican. Unlike some political organizations, NRA members tend to vote their gun rights first.