You can't pass Sharia law unless the majority of people in the country are muslims because Sharia law makes non muslims second class citizens. This is why in Sudan which had a very solid Christian population in the south they terrorized, raped and pillaged them to get them to leave so they could pass Sharia law.
They would need to get millions of muslims pouring into this country while also terrorizing Christians to leave, (who knows maybe make crime skyrocket, empty the prisons, open the border, who knows). Even then you still have to kill off millions of Americans (how would you do that, unleash snakes into the whole country?)
So no one is claiming that they are trying to do this.