With mid-terms coming up in 8 months, no liberals are going to support taking away the safe zones in schools (one of his conditions), and fence-sitters will see it as a good idea. The right won't easily let the age to buy increase to 21 because it erodes the 2nd amendment, and has constitutional ties to voting age. Trump shot down multi-state concealed carry (for now) because that eats into state's rights.

President Trump wants everything in one bill, which will suck up a lot of time and effort, stalling it past November. The election will be about gun grabbing, which will bring out conservative voters in droves, while liberals won't have such a big rallying point.