What makes you think that securing the border (which is code, by the way, for keeping the brown people from the south out) is going to help with the problem of fentanyl in our heroin supply? That fentanyl is coming from China in small packages. Picking up on those packages as they cross the border is basically impossible, given the shear number of packages that come in from China everyday. Given that drugs like carfentanyl are more than 300 times as potent as heroin, small packages are all it takes. A kilo of fentanyl is like half a ton of heroin. How do block that? It's not like heroin itself, which needs to come in by the ton. I order shit from China all the time, and I've never had a package inspected. They couldn't possibly inspect enough packages to detect a significant fraction of the incoming fentanyl, even if they really, really wanted to. Republicans being in office isn't going to change that. The only thing that could would be to add significant tariffs to reduce the amount of incoming stuff from China. But that would be pretty harmful to our economy, so it definitely isn't happening, no matter who's in charge.
Realistically, the only way to deal with the problem of fentanyl contamination of our heroin supply is to create standards for heroin purity and potency. This, of course, would require legalization, which is the right answer. We could actually do without legalizing heroin, by legalizing narcotic replacement therapy, and making it freely available. Note that this doesn't mean methadone or Suboxone, although those two are fine alternatives. It means prescribing something like oxycodone or morphine to heroin addicts sufficient to keep them from going out and buying heroin. Neither the R's nor the D's are currently up for doing that though. The D's are a bit closer with wanting to legalize pot. Baby steps.