If I was Trump I would just go ahead and start campaigning against Biden and pretend Haley doesn’t even exist (because for electoral purposes she doesn’t). Don’t respond to anything she says, just utterly ignore her.
The trouble is, by getting Democrats to cross over, she is stealing delegates that should be Trump’s. These are people who intend to vote for Biden in the fall, but temporarily switch registration to vote for Haley, just to weaken Trump.
In NH, the split was 10-12…. with only genuine voters it would have been 3 - 19.
They will do the same thing in SC and other states - it will give the false appearance that she is a contender. The fake news will reinforce this narrative. Trump might be forced to debate her if she earns a significant number of delegates.
Nikki Haley is allowing herself to be used by the Democrats in this manner.
The GOP needs to tighten up the rules in open primary states to stop this from happening.