I’m going to fight the premise of the OP. To the extent that people in or running for elected office on “the left” have been calling out the Trumpists for fascism, it has been using the word fascism, but light on specific acts. I haven’t heard any major speeches from Joe Biden, Chuck Schumer, Hakeem Jeffries, etc. laying into the Trumpists with specific detailed accusations rather than just the word fascist. Here are the sort of specifics accusations which if made I’m certain would be responded to.
Chuck Schumer, directed at Tommy Tuberville. “You are going against the will of the American people. You’re trying to weaken the United States military by blocking the appointment of qualified people who want to serve their country, so that Trump can instead install officers who are more loyal to Trump than the United States. You are trying to turn the US into a fascist state by trying to take over the military from within.”
Georgia Democrats running for elected office (I’m excluding Fani Willis, who I think is doing an excellent job) directed at the 2020 Republican electors from Georgia. “You claim to be following orders from Trump, but that’s what happens in a fascist country, not a democracy. As potential electors from the state of Georgia, your boss was the voters of our state, not Donald Trump. By following his orders, rather than the will of the people of our state, you show that you care more about installing a dictator than honoring the will of the people. You’ve betrayed your country and should be considered traitors to the United States.”
Any Democrat running for senate against any of the Republicans who voted to acquit Trump, to that incumbent senator. “You voted to acquit a man who wanted to overthrow the US government for his own personal benefit and make himself a dictator. That’s what fascists do. By voting to acquit Trump, you in turn have also betrayed your oath to uphold the Constitution of the United States, not to swear fealty to Donald Trump the man.”
Until Democrats start saying things like that, of course Republicans can ignore some mealy mouthed generic mention of fascism that’s included in some otherwise dull boilerplate campaign speech.