The 2019 Election result. We on the left have to stop ignoring it just because it is inconvenient. This is the pertinent outcome. It doesn't really matter about anything else. You can argue all you want about the minutiae of the Jewish issue.
I don't think Corbyn is an anti-semite personally, but some around him are so anti Israel (whose treatment of Palestinians is indeed disgraceful) that they certainly appeared so and the messaging from Corbyn fostered this, gave tacit approval and his leadership or lack of it led to many Jewish people leaving the party and many in the public to believe the Corbynites to be anti-semitic.
Technical arguments otherwise are irrelevant. The country gave it's verdict and it has resulted in an 80 seat majority for the most right wing, ideological, incompetent, government in UK history. That is the reality. It is fact. Stop denying reality and face it.
The left need to understand the mindset of the country.
Labour never wins when it is divided. Look at the record. Look at how we lose and how we win.
Ramsey MacDonald managed to get elected PM, but then the Left of the party decided he wasn't left enough and couldn't countenance working with other parties in a National Government during the Great Depression. Labour went from a 288 seats (to the Tories 260) minority government (with the support of Lloyd Georges liberals) to MacDonald winning the largest ever mandate from a British PM with 554 seats. Unfortunately 473 of those were Conservatives, 68 Liberals and just 13 National Labour. The Labour Party under Arthur Henderson won just 52 seats. The country wanted unity and moderate, centrist policies.
If the Left want to point to Atlee, they must remember the context of Atlee's win. He won the 1945 election with what was for the time a progressive programme, but it largely followed the Beveridge report (who was actually a Liberal), it followed the most momentous event in history, after every single serviceman had been sent the Beveridge Report by the M.O.D and spent two years digesting it, discussing it and dreaming about it in their foxholes, bunks and messes. It became
the reason they were fighting, that better future, not for empire or to defeat Nazism. Labour were going to implement it. Churchill wasn't.
Then we had the post Atlee period when Labour were riven between the Gaitskillites and the Bevanites and were out of power from 1951 until Wilson brought everyone together again with a more centrist compromise. Wilson was elected PM twice.
Next, when Labour went down the Michael Foot route on the left, the country rejected them. In fact Kinnock had to spend years fighting to oust Militant to get the party electable again. He narrowly failed in 1992, largely because of Major's personal appeal as a moderate to the nation.
Step in Blair. Three times centrist Blair was elected and he did a hell of a lot more for workers, the NHS and Education than Thatcher before or Cameron, May and Johnson since.
There has been a lot of talk in the last 5 years about respecting democracy, largely disingenuous guff from brexit supporting pseudo democrats, who either had no intention of delivering the promises on which the votes were cast, or were so monumentally arrogant and stupid they didn't understand they couldn't.
The population of this country is by and large moderate, conservative with a small c. Even Marx knew that. You know what, veering between centre left and centre right is not a bad thing. The Left need to understand that sometimes the Right are right about some things and sometimes the Left are wrong.
Also, it is fundamentally undemocratic to achieve power promising certain things and then use that mandate to deliver something else, as Milne and co. tried to do. We can see this with Brexit. We can see how the bluekip take over of the Conservatives has turned them into a very different beast, with the purge of moderates throughout the party. We've also seen huge push back by the country as a whole on their more ideologically driven policies and decisions and what we are also seeing is grassroots progressive alliances and tactical voting, leading where Labour refuse.
Labour need to take a leaf out of the Tories book. The Conservatives are ideologically driven, but their main driver is staying in power, so they will adapt and move to the left when they are in danger of losing power, just enough to retain it. What they understand is that they will always oppose the Left, whether in opposition or government, but you can oppose it a lot more effectively in government than when you are not in power.