That's bullpucky. Trump is not pro-2A, and the bump-stock extension was passed under his administration. As with most things, his position is vague, but mostly catering to monied special interests, as evidenced by how his administration approached 3D printing (the gun industry can't make money if people can print their own guns at home). In any case, he's certainly not been a vocal 2A defendant, it doesn't come up in his rhetorical rants, and his record while in office was wishy-washy at best. This hasn't made a single iota of difference to his base.

2A is a key issue for many people, but it's specious to claim it's the issue that's deciding a large number of flippable votes. NRA symbols are usually stuck next to a Confederate flag sticker, and that tells you all you need to know about that topic.