The current situation with abortion will pose a threat similar to the slavery divide. There's a clear division of states which allow it and those with bans. Some of the states with bans are threatening prosecution of women who cross state lines for the procedure.
There's a similar situation coming up regarding gay and trans rights. Some states will continue having equal rights. Other states may not. This situation will get even worse if the Supreme Court overturns gay marriage and sodomy law precedents.
All of this will create even further division. One of the harbingers of a coming civil war will be internal refugees and increased internal migration. If people start fleeing the red states in noticeable numbers, then that's the time to start getting worried. Especially if some of those fleeing make their moves suddenly. If you start seeing hundreds, or even thousands, who leave their homes in the middle of the night to escape, then you know something bad is coming.
There's already a trickle of such people. A few in Texas did just that. Parents of trans kids who wanted to get away from a situation where the state posed a threat. And they felt like they had to leave immediately with no notice to anyone. We'll start to see more of this if some states decide to ramp up prosecutions of women and investigations into miscarriages.