I don't see why "primarying" should be considered some kind of cruel punishment reserved for the worst politicians ever. If the party's voters want someone other than the incumbent to be their nominee, they should be able to make that happen..
The problem is of course, that this can backfire. Namely, that if you primary, and lose, then the congressman finds will get the idea that they have even freer reign to be right wing/terrible, because you've shown your hand, and they've proven they can beat it. Furthermore, it sends the signal to other questionable congressmen that they don't need to worry about a successful primary challenge. This is why if you are going to primary someone, you need to make sure to win. And one thing Democrats literally never win is a primary challenge. And even when they win, they still manage to lose (ex. Lieberman).
Also, this is just my view, but there are maybe 10-15 issues that I'd prefer to put as litmus tests over this. If the deal somehow got blocked maybe it would have been different, but in the end, nothing bad actually happened.
It's because a good chunk of the left (the base, not the actual politicians) is deeply anti-semitic and views Jews as inherently suspect in terms of loyalty. That's why politicians like Casey or Barrow are allowed to differ from Democratic orthodoxy in terms of major issues, but the second Jewish politicians dare to differ from the President - by taking a position long held by the party before this President completely altered our foreign policy, mind you - the calls come for their heads.
No one on the left wants all Jews to move to Israel to be slaughtered for the end times. Only people in your party want that.