First, I want to point out a few things.
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Third Party voters, ie the Greens and PSL, swung 0 elections. This is a hard truth to look at for Democrats, ultimately it is unlikely that the genocide in Palestine made a large difference in the Harris campaign.
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I would not consider Bernie or AOC to be "far left," they are moderated by European standards. If you mean within the US government, then that makes more sense, but the US is an outlier globally with how right-wing it is on average, so that bit confused me.
To answer your question about Palestinian liberation, it began when Palestine was first being colonized, back in the early 20th century. Leftists all over have historically supported Palestinian Liberation this entire time, it is only as salient of an issue now because we can watch it on Social Media.
A book titled The Ethnic Cleansing of Palestine, documenting the genocide in Palestine since the mid 20th century in detail, came out in 2006. This has radicalized a lot of the western left in the modern era, but this is by no means the first time.
The Marxist-Leninist group Popular Front for the Liberation of Palestine was formed in 1967, and was supported by the USSR. Incidentally, they called for a boycott of the Democrats and Republicans in the 2024 General Election (link to a translation from Arabic). They have been calling for a secular, single democratic Palestinian state to be occupied by former-Israelis and Palestinians alike, with full equality between them. This has been elaborated on in their 1969 book Strategy for the Liberation of Palestine.
Even then, the PFLP is just one group in Palestine, Palestinians and Leftists abroad have opposed it since the beginning. Albert Einstein wrote a letter to the New York Times in 1948 warning of fascist elements within the newly created state of Israel, and their treatment of Palestinian villiagers.
The fundamental difference that you are seeing is that the far-left has always opposed the Palestinian genocide, be they Communists, Anarchists, even moderates like Social Democrats. The US government, however, has strong economic ties to Israel, as they help assert power in the region, which allows the US to cement the Petro-Dollar as the global main currency. This means that within the US government, politicians are incentivized to minimize the appearance of genocide.
Free Palestine has been a movement for a century, it is only a more salient issue because the genocide has escalated in the last year and it is in full-view on social media.