Asking lefties to side with Israel isn't going to work for several reasons, but the main ones are :

1) Israel started out as a "free" communist state (the Kibbutzim system) and over a few decades the people en masse freely abandoned it to create what pretty much is the cutthroat almost purely capitalist society that it is today.

It is the poster child for the idea that voluntary socialism doesn't work. It's existence at this point is an offence to the ideas that guide the international leftist movements.

Israel's evolution is the reason that socialists worldwide aren't satisfied to create the option for people to be socialist, but want to have the state force socialism on everyone.

The fun thing is, there is one single thing islam and the west agree on, and that's cutthroat capitalism. In fact, islam's sharia is significantly more cutthroat than western capitalism, including for example slavery. You'd think lefties would have a huge problem with an extreme capitalist party. Hell, you'd think Americans would have more problems with religions pushing slavery.

2) Israel creates a hard-to-deny "western ideology" idea of superiority (not that Israel is all that western). Compare any property of Israel. Without resources, without much area, without much means it has achieved so much success it's not funny :

(a) a democratic state, surrounded by dictatorships (Egypt and Saudi)

(b) large, functional cities with an actual economy in that region (meaning not just foreign contractors drilling for oil and slaves, or just a sea of people wholly dependant on foreign food aid)

(c) the population actually has decent lives

(d) freedom of religion (maybe this ought to be higher up the list). Actual significant, prosperous pockets of pretty much every religion.

(e) can actually survive without external sponsorship (e.g. the US pays more to a Gaza inhabitant than it pays to an unemployed person inside the United States ... that's the worst example, of course, but the US pays per-capita recurring amounts for every person in every country bordering Israel except Syria and Lebanon (where it's mostly done by other states).

3) Israel is an easy scapegoat for the very bad relations between the west and islam. What everyone forgets is that there has only ever been open war between the west and islam for ~1400 years, and then low-level terrorist/insurgent warfare in the 20th century that's easier to ignore.

Israel is obviously not the cause of this conflict, but it combines this unfortunate position for both parties :

(a) for muslims it's the ultimate offence, an open, country-sized, big font note on every map saying "western freedom-of-religion culture is superior to islam !". A western state controlling one of the "holy" cities of islam. The central "reason" for islam is that allah promises muslims will conquer the world. Well, they can't even conquer Jerusalem ... They literally are unable to impose sharia law on one of the places it was created.

Needless to say, muslims give zero consideration to the fact that it's also a holy city of Judaism and Christianity.

The crime of Israel, in the eyes of moslems, is not the victims it's defensive campaigns caused, it's not the lost territory, it's is that it shows you can resist islam, right in the middle of it's heartland.

Think of it like if there was a successful communist mini-state controlling a few neighbourhoods of Washington DC, that the US constantly, unsuccessfully, tries to sabotage using any and all means. That's how Israel appears to middle eastern moslems.

(b) for western states it's the demonstration of what is needed to live with moslem neighbors. That the situation is effectively unchanged from the 19th century : the only way to survive moslem neighbors is to constantly be ready to start a military campaign at the drop of a hat.

The moslem way of fighting makes following human rights, the separation of civilians and soldiers, nonsense. Islam dictates that every moslem is a soldier, or has to contribute to the war on "dar al harb", meaning they have to contribute to fighting some non-muslims. The big problem this causes is that moslem soldiers pose as civilians (because that's how "the prophet" fought), then suddenly take out weapons in a crowded marketplace and start fighting.

Contrast the Christian (meaning canon law) way of fighting is : you send an envoy to the enemy, stating "date such-and-such, we fight here-and-here, we will not touch your forces in area A, you will not touch our forces in area B, and whoever wins gets to take city X", this is negotiated until both parties agree and then hostilities open. Surprise attacks against civilians, raids and insurgent attacks, moslems' basic strategy (whether you're talking about the prophet's wars or recent stuff), is utterly out of the question and considered a moral abomination.

Obviously canon law is about as successful regulating war as sharia law was, or any other law really, once the guns start blasting, but it generally does govern the start of conflicts. The best example of "but the law says" versus warfare in my mind is the Spartacus campaign in the Roman Empire. That's a story that you start reading very hopeful, everyone starting with good intentions, everyone out to improve everyone's life and everyone agreeing to abide by a legal system (even if that legal system is somewhat ...) and you are literally terrified at the end. It doesn't happen because one side behaves badly (they do behave badly, of course, just for good reasons). Everything that happens is sort-of reasonable given the situation, yet it ends in a torture massacre, and a global repeal of some freedoms in the Roman Empire.

This means that military bases are the central feature of every moslem town, and everyone is a soldier. It means schools, mosques, shops and rocket launching facilities are one and the same thing. (Mosque, incidentally, translates to "fortress". Historically mosques were fortresses first, and had several other functions. In historical mosques you will find markets, slave markets (recognizable by the fashion-show like podia), weapons inventory, food inventory, stables, schools, soldier's bunk rooms, walls, siege weapons, ... The prayer room (masjid, not mosque. There do exist masjids that aren't mosques, mostly in non-sunni brands of islam).

Western states do not want to get into this fight, and Israel is a constant reminder that if one side in a conflict decides you're in a fight, then you're in a fight. If one side decides to destroy some legal right, there is a massive cost on the other side to maintain it. Freedom of religion, civilian versus military separation, economic freedom, freedom of movement, immigration, ... all of these cannot be sustained in a real conflict. The west simply hasn't gotten in a real conflict for more than a generation, so nobody seems to remember what happened to legal rights in America during WWII.

In strategic reality, Israel is a lightning rod : it's getting attacked by moslems, who would otherwise be attacking other things (they have for 1500 years, never stopped). In public opinion, it's a stark and public reminder that there has never been even a single decade of peace with the moslem world, there have only been periods where muslims were utterly defeated in most regions, with conflicts limited to border regions. It's a reminder because it's an offence to moslems and so moslems won't stop attracting attention to it, where the conflicts islam gets into everywhere else, Sudan, Mali, China, Pakistan, India, Azerbajan, Iran, Eastern Europe, Yugoslavia, Zanzibar, Kenia, Somalia, ... sort of fade into the background.

What people don't realize is the stakes of this conflict : if the conflict escalates (moslems don't have to win to achieve this) much of the 20th century social advances will have to be at least temporarily reversed. Since the alternative is destruction, they will be, because when it comes right down to it, survival and freedom from raids is more important than pretty much anything else, including things like due process, democracy, ... There is no way islam's strategy can work, but it can destroy a lot before it is defeated. Sadly, if the conflict escalates, some of these rights will be lost for a long time.