Trump starting his own party might be the best thing for the Republican Party. They have too many crazies at the State level and Trump would siphon off a lot of them. The Texas GOP have endorsed QAnon. Oregon GOP is trying to pretend that the insurrectionists at the Capitol were antifa - even though the vice-chairwoman of the Oregon Young Republicans is one of the rioters arrested. Hawaii GOP have promoted QAnon and a Holocaust denier.
It will hurt, in that it will cost them votes and power, but the alternative is to sell the soul of the party out to lunatics who want to destroy freedom in the US.