My guess is that any other Republican president, be it Romney or McCain, or any of the Bushes, would have backed down by now, even as they know that border security along the southern border is a relevant issue. Probably they all would believe in border security as a principle too, but when faced with opposition they would do the milquetoast blink.
The MSM wing of the Democratic party would crank the screws and remind everyone that starving the government of budgeted funds would be a losing election issue for them, and, pragmatism over principle, these mainstream Republicans would all back down, every time. They would back down on holding funds for abortionist PP and they would no doubt back down on forcing this issue too.
Trump doesn't give a flying fiddlers about all that. He doesn't back down, but doubles down on the craziest rhetoric even.
Other politicians would be called evil and react with shame, and do what it takes to take on the persona of a good boy now.
Call Trump evil, and he shrugs, and asks the provoker to come up with some new material that everybody doesn't already know.
Maybe it is the case that Trump is totally unprincipled on any issue. No matter. Whether the issue is a matter of principle for him or not, it is the principle of his base, and a goodly proportion of the American population, who like the ideas of laws protecting even foreign little girls from being the sexual playthings of coyotes running the border.
Trump is loyal to that American demographic that wants those laws enforced, and he is loyal to people who are loyal to him.
Dems can go the Rotherham root and consider diversity to be a bigger priority than the safety of little girls having their bodies being used by criminals. That is a thing now for the left now globally, from England to Cologne in Germany, to the southern border of America.
Trump is loyal to the principles of his base on this issue, and he is taking a stand. That is something that most other Republican politicians have forgotten to do when it comes to actual principles. They have always follow the political winds as forecast to them by a hostile leftist media, instead of their principles.
Trump shoots his mouth off in a hundred different directions at once, but, win or lose, when it comes to being loyal to the principles of his base, he remains steadfast.