It’s probable that portions of Melania Trump’s speech at the Republican National Convention were lightly rephrased versions of passages from a Michele Obama speech. If this was done by a speech writer, Melania is not really guilty of anything. If it was done by Melania, it suggests that maybe her view of the Obamas is more favorable than that of her husband, i.e., if she drew from the speech so much, she must have really liked it on some level. In either case, Hillary would be advised to cut her some slack by, say, issuing a statement that she doesn’t doubt that the speech expresses the sincerity of Melania’s belief in her husband.
The key point has already been made–that this incident is yet more evidence that the Republicans and the Trump campaign are in disarray. To keep the issue alive serves no useful purpose. I say this not because I have a fondness for models (even a model from a former non-Stalinist Slovenian communist family), but because it’s the smart thing to do–it helps to show that Hillary’s the adult in the race who wants to keep things focussed on what matters.
I suspect the plagiarism is just the result of a lazy speech writer and a careless campaign manager who never vetted it before it went primetime. The real question is whether exposing this will do damage to Trump. I think the answer is it will make little difference in moving votes away from him. The zealotry and idolatry of the Trump supporters is such that he could get arrested for torturing kids in his basement and they would find a way to dismiss or deny it. However, if this plagiarism was really a wedge issue, I believe it would be entirely appropriate to use it. Plagiarism is not a trivial issue.