Republicans have been manufacturing outrage at the border to score political points on a regular basis, like that migrant caravan which convenes every four years like a Fox News political convention.
However, this recent crisis is a bit different, as we have numbers that show that there have been an increase in asylum claims at the border vs. before the Pandemic, which is overwhelming the courts that handle this sort of thing. If someone makes it over the border and claims asylum, it can take years to get a hearing, and in the meantime they can live and work here. That was not the intent behind the asylum process.
Some Senators on both sides realized this, and used the opportunity to try and fix the asylum process. Yes, it also reduces the types of people who could claim asylum. But it also addresses critical staffing shortages in CBP and other agencies who have to deal with the influx of migrants on a daily basis.
So, this is an evidence-based attempt to make things better. So, of course, Trump is against it, because using immigration as a campaign plank matters more to him than fixing the problem. As much as he complains about it, having the problem to talk about is more valuable to him than fixing it.