This is not leftism. What you're proposing is just a shiny new liberal party. This hypothetical new party would be subject to the same mechanisms and forces that corrupted the democratic party. Give it 50 years, and we are back at square one: capital will swallow any agency that your proposed party once exhibited.
I really encourage you to do some reading on Marxism and leftism before you throw around the term so carelessly. https://socialistworker.org/2012/07/20/what-is-a-vanguard-party
Decrying actual leftists for not participating in american electoralism that is wholley orchestrated by capital is going to fall on deaf ears. Leftists have no interest in creating a new capitalist party and subsequently refining capitalism.
What are you on about? Democrats, in one of the most embarrassing defeats in recent history, literally just made it clear to the world that they would rather lose than cater to leftists. Biden decided that carrying out genocide in Gaza was more important than defeating Trump.