"In fact, the only real change on this topic is on the Left, who since Trump took office decided illegal entry into the U.S. no longer matters."
There were less border crossings under Obama, than under Trump. Also, given that there are more over stayed Visas than border crossings, why isn't Trump going after those?

"Painting Trump as Adolph Hitler -- a common response on the Left -- because he wants a strong national border is also fearmongering. "
I'm not arguing for open borders. I'm for borders. But doing it for political points is the wrong reason. And if a border is so damned important to him, why isn't he putting one on the north side? People cross illegally there too? The drugs he's against come into this nation through legal points of entry, boats, and planes, where is he on that?

"is since considered an "international criminal gang" "
So is the Mafia, and not a word from Trump...

"But I don't have reason to doubt that some of its widespread members do enter the US illegally."
In reality, MS-13 members make up a fraction of Border Patrol arrests and a small part of gang activity in the United States. While the group has committed brutal murders on Long Island, there is no evidence that the gang is increasingly sending members into the country. MS-13 members, some 10,000 in number, make up less than 1 percent of the approximately 1.4 million gang members in the United States, according to F.B.I. estimates. And Trump is silent on other gangs... Far from menacing cities across the country, as Mr. Trump has suggested, the gang’s presence is concentrated in Long Island, Los Angeles and the region outside Washington.

In addition, most MS-13 recruits are not migrants but teenagers who live in the United States and are alienated from their communities, said José Miguel Cruz, the research director at Florida International University’s Latin American and Caribbean Center. In those areas, the gang may be the only group that provides a sense of identity, he added. Mr. Trump’s statements conflating immigrants with barbaric “thugs” are misleading. Among undocumented immigrants convicted of crimes who were apprehended by Border Patrol, relatively few were convicted of violent crimes such as assault and homicide. Research shows that more immigration either reduces crime, on average, or has no effect on crime rates. While MS-13 began in the 1980s as a small and unorganized street gang in Los Angeles, some evidence suggests the group expanded in Central America after the United States began deporting illegal immigrants — many with criminal backgrounds — with greater intensity in 1996. Experts have described this process as “exporting” American-style gang culture to Central America.
MS-13 has entered the national conversation because of Mr. Trump’s anti-immigration agenda, not because it has become more threatening.

"Just when we most need a boost of calm dispassionate reason"
The right is just as guilty.

"I don't see how I'm "still go with fearmongering""
You are still espousing (mind you, not as doggedly as the right, which I do respect you for) that the border is hell, a war zone, a Major issue... when it is not and has not been for a long time.