"Building an alternative" doesn't happen in the ballot box. It happens everywhere else.
It happens by a better voting system rather than FPTP, for which I'm doing actual, active advocacy. (Are you?)
It happens by working at a grassroots level to get people with better opinions elected, all the way down to local judges, city council members and library boards, where I, once again, am active. (Are you?)
It happens by getting involved in politics at a local level and building a movement. I'm doing that. (Are you?) It doesn't happen by throwing a tantrum in the voting booth.
The fascists know this. The fascists use this to their advantage. And the fascists would absolutely love for there to be 10 competing leftist parties acting as a spoiler effect for liberals. Because as bad as liberals are, fascists are worse.
Throwing out a "no u" when I point out how the things you are doing are paving the way for fascists is not a good argument unless your goal is to actually get fascists into power. And I will choose liberalism over fascism, because that's the harm reduction path to leftism, whereas letting the fascists win is the harm maximisation path.
You sure can if you believe that making an insignificant point in a ballot box is worth more than the actual lives of people who would die because of a Trump administration but not under a Harris one. But if you want to make an actual difference. the ballot box is one of the very few times you need to hold your nose and do the uncomfortable thing of choosing liberalism over fascism.
But if you're okay with fascism, sure. Go and make your vote a spoiler that helps the fascists win. I'm sure the people who die because doctors who were scared to provide medically necessary abortions will be grateful that you did the morally superior, but entirely ineffective, thing.