Leftists of the Socialist and Marxist persuasion oppose Zionism not because of claims of genocide or antisemitism, but because it is reactionary, Nationalistic movement that is detrimental to class unity.

The Bolsheviks and Lenin fought furiously against antisemitism and put an end to the Pogroms that were an artifact of the Czarist regime prior to the Russian Revolution in the early 1900s. Jews had representation among the Bolsheviks (The Bund).

Here's an early recording of Lenin addressing antisemitism, and condemning those who foment hate against the Jews:

https://www.youtube.com/watch?v=C_n_qtgUKnY

Here's Lenin's analysis of the The Bund, where he addresses the propensity of some in it to focus on Nationalistic Zionism rather than working class and party unity:

https://www.marxists.org/archive/lenin/works/1903/oct/22a.htm

An excerpt from the closing paragraph:

Instead of proclaiming war on this historically evolved isolation (further increased by the general disunity), they elevated it to a principle, seizing for this purpose on the sophistry that autonomy is inherently contradictory, and on the Zionist idea of a Jewish nation. Only if it frankly and resolutely admits its mistake and sets out to move towards fusion can the Bund turn away from the false path it has taken. And we are convinced that the finest adherents of Social-Democratic ideas among the Jewish proletariat will sooner or later compel the Bund to turn from the path of isolation to that of fusion.

Marxism and Communism is not antisemitic, and in fact the Bolsheviks counted among their party membership many Jews. Marxists are opposed to Zionism however, because forming reactionary and imperialistic nation-states along religious or ethnic lines is completely anti-thetical to the anti-imperialism of Marxist theory. The Soviet Union waged a campaign against Zionism, because it correctly identified it as a product of capitalistic imperialism.

This is one main reason why Zionists are so insistent on blurring the difference between Judaism and Zionism. They wish to promote their Nationalistic reactionary ideology as one and the same as Judaism, so they can attempt to smear anyone (even other Jews) who don't agree with as antisemitic or 'self-hating'.

This idea that the Soviet Union was founded on antisemitism or 'blood libel' against Jews is patently false. As evidenced by Lenin's speech and writings, he was emphatically opposed to antisemitism and worked to end the pogroms and antisemitism of the Czarist regime (which was eventually toppled in the Russian Revolution).