Interesting question. All of the above, of course. But I wouldn't blame the rest of the GOP field, though, just most of them.
The most immediate threat is illegal immigration. It's existential. Trump has put it first. Will he continue to grasp and act on the fact that we must be decisive and cheerfully do what is (to liberals only) unthinkable. If he's truly for "touchback amnesty," he doesn't get it.
The second-most important threat is Moslem invasion. He seems to get that, and in the right priority.
The over-arching threat, which is both existential and eternal, is the destruction of our culture caused by immorality and the lack of religious sense and self-discipline. This has led to the destruction of both family life and the economy. In a nutshell, young men with a sense of purpose and families to feed don't hang out playing video games in their parents' basements. These young men inspire young girls to look forward to marriage. These young men sire legitimate children who know how to work, love, and pray. Such children, our real future, are being born in certain enclaves, but it should be happening all over America, and it's not.
The last threat is the ultimate cause of the first two. Does Trump get that? Does he have the attention-span to even grasp it? I hope he does. May his Presidential journey make him realize it, and be the leader he should be.
I agree completely. However, I think there a deeper contributing problems to illegal immigration and illegal governance that promotes illegal immigration. For instance, and I think you see the same thing, our birth rate is below the replacement rate. This creates a vacuum in our welfare state/consumer and debt based economy. It also creates recruitment problems and military policies that deploy an army to recover a snowflake like Bowe Bergdahl and spends 850,000 to 1,400,000 to deploy 1 11B downrange for 1 year.
Whoever wins the election better figure out how to get more natural born American mothers to naturally breed some more natural born American children instead of trying to get through Ranger School or UDT. Otherwise we are going to get overrun like Germany.