Yes we can
It's against the law, and we have the right to enforce it.
And that "enforcement" is what caused the Civil War.
If the states want to leave because the legislature and the executive branch can't do their job, fine, let them.
By enforcing this "law" you are basically declaring un-provoked war on a new country that didn't even spit in your direction.
Also, yes it is legal.
It is woven into our constitution and has been a philosophy of government since the late 1700's and the early 1800's: If the government does not fit the needs of the people, the people have every right to overthrow it.
that includes secession