I hope you are joking. Why would it be illegal or unethical for someone....anyone....to attempt to persuade voters about...anything? That IS democracy at work. I am not addressing the political wisdom of the issue...good arguments could be made either way. But why in the heck would you think "persuading" voters is problematic in our free Republic?
This isn't "persuasion," it's an attack, and it's important to make that distinction.
Is it Illegal? No. Is it unethical ... yeah, for a president allegedly of the same party, it's unethical. Vindictive? Damned straight. Silly? Definitely.
And is it politically suicidal? Yes, for two reasons. First, it shows that the GOP cannot count on Trump's support, and that goes both ways. Second, it's ridiculously stupid to split off a group that represents the difference between majority and minority for the GOP.