According to Daniel Greenfield :

"Trump wants to make Cortez, Omar and Tlaib into the public face of the Democrats by attacking them, forcing the Dems to defend them."

That puts so-called 'mainstream' Democrats in an untenable position, because "The more outrage is stirred up, the more they become the politicians most associated with the Democrats. And, it should go without saying, this does nothing good for the Dem e******n prospects in 2020".

https://www.frontpagemag.com/point/274314/trump-making-omar-cortez-and-tlaib-public-face-daniel-greenfield

All he has to do is wag his finger and they pitch another hissy fit, and he's using their own radicals against them. Smart Donald. Very smart.