Finally, we will pursue aggressive criminal or immigration charges against anyone who lends material support to terrorism. Similar to the effort to take down the mafia, this will be the understood mission of every federal investigator and prosecutor in the country.
Trump thus implies that DOJ is not currently as aggressive as it might and should be when it comes to material support prosecutions. What baloney. Ben has already made the point, but I think it's important to spell this out at greater length.
In the aftermath of 9/11, President Bush made it crystal clear to Attorney General Ashcroft that he was to do all that he could to prevent further attacks inside the United States, and Attorney General Ashcroft in turn made sure that DOJ prosecutors made aggressive use of all legally-available tools. Among other things, this meant making significant use of the 1994 and 1996 material support statutes (which previously had not seen much action). This resulted in a wide array of prosecutions in the years that followed, some of them arising in circumstances that raised challenging questions about how aggressive enforcement of these previously-neglected statutes had become. At any rate, no one at the time would suggest DOJ was not leaning in hard enough on 2339A and 2339B prosecutions (or, closely-related, prosecutions under IEEPA or section 956).
Fast-forward to the era of ISIS. Can one claim that the Obama-era DOJ has taken its foot off the pedal at just the wrong time? Not remotely. While I reject the idea that you can make apples-to-apples comparisons between the post-9/11 period and the ISIS-period simply by toting up the number of cases (since there is no reason at all to believe the both eras offer the same number of occasions when material support arrests/prosecutions reasonably might be pursued), even a casual review of the record shows that FBI and DOJ have been extremely active on this front.
So what could it possibly mean to imply that a Trump DOJ (wow, it's hard to write those words...it's as if the keyboard itself finds them unnatural) would be more aggressive? One possibility is that there would be a systematic effort to arrest suspects at an earlier stage. What a risky idea, however; this would cost the government opportunities to collect intelligence about other possible ISIS supporters, while also increasing the chances of a failed prosecution (not to mention enhancing criticisms along the lines of precrime). Another possibility is that it would mean an increase in reliance on undercover agents and confidential informants. But, to be blunt, we already are doing a tremendous amount of that sort of thing, and it's rather hard to see room for a surge. Of course, another possibility—the one Trump mentions in the second sentence above—is that it means conveying to prosecutors that this is in fact a top priority. But that is already the case, and has been for a very long time.
In my opinion—and I've been closely tracking the use of 2339A and 2339B since the late 1990s—it is very hard to see how FBI and DOJ could be more aggressive on a systematic basis (nevermind what could be said about any one particular fact pattern) in the use of the material support statutes, at least not without incurring unreasonable costs both to security and liberty.