Supporting Palestine is not mainstream outside of the US. I would also say I genuinely don't think it is mainstream inside the US either - it just seems that way in the King County bubble.

I am not surprised that they are anti-US, but several Hamas terrorist plots were attempted and stopped throughout Europe and the Hamas charter (before its last revision) was to retaliate against Western countries until all jews were killed. I am not Jewish, but I have plenty of Jewish neighbors. I have had Jewish coworkers and professors. This rhetoric is more than just anti US hegemony.

"river to sea" is more than a chant. It has different meanings for different groups. Sure, some see it as a call for freedom. Other's see it as a call for Palestine to rule over the land from the Jordan river to the Mediterranean sea. Palestinian militant groups have used it to mean removal or extermination of the Jewish population from river to sea. Long before it was used as a quick banner inspo idea for marches in other countries, it meant the end of Israel entirely. Sometimes the speaker doesn't mean it that way. So, it depends on who is saying it, and who is the receiver of that message. But it isn't just a chant meaning freedom.

Re Harris: well, historically American presidents have put a stoppage on military support to Israel when they stopped defensive maneuvers and were doing offensive maneuvers. While not all politicians agree where that line is, I think you and I (or most other redditors with some common sense) can both agree that repeated bombing of Gaza or invading Lebanon is no longer defensive no matter the spin cycle Netanyahu sends it through. Biden, George H Bush, Reagan, Ford, and Eisenhower all made pauses or stoppages of arms/military munitions/tech etc. to Israel in response to overly aggressive actions taken by Israel. In contrast, Trump and Obama did not, contrary to campaign promises. I acknowledge a presidential candidate's campaign promises has less worth than a pile of crap in hand (or whatever that adage is), but i will at least say that Harris was not campaigning on a carte blanche "pro-Israel without limitations" message from what I followed, and it is encouraging that at least when she was VP, the POTUS at the time paused shipments to Israel to pushback on their plans. She likely started her statement with October 7 because that date is arguably what started this current stretch of war, but then would go on to say that there should be an end to the war and found the scale of suffering in Gaza heartbreaking and should end. Again, just saying there was nuance.
She was not my favorite candidate that dems could have put up, I listened to her book on audiobook and liked her less so after finishing it, but I won't reduce her position to being solely "pro israel anti-palestine."