Trump is right that the 2A won't be repealed, but the Heller decision could very well be overturned by a SCOTUS packed by a Dem President. Justice Stevens is the author of the dissenting opinion in Heller, and took the position that the 2A was merely intended to support a militia, not an individual RKBA.
The 2A isn't in peril, but the Heller decision is. The individual RKBA relies on a fragile SCOTUS majority, and just as conservatives demand Roe v. Wade be overturned, so do liberals demand that Heller be overturned. Think about that reality when you decide to indulge your vanity and stay home rather than vote for a Republican.