Since I'm a professor, I can't resist this one. I've caught and "prosecuted" plenty of cases of plagiarism. To be blunt, it's usually the dumb ones who get caught, so take that for what you will. For those who want the details, I won't bother retyping it-- here's a link. I'll simply say this: I've nailed students for more subtle stuff than this, and I take special pleasure in it. Will it matter? No. There's a long and proud tradition of plagiarizing politicians, and it's worth remembering that the whole "you didn't build that" line was a botched attempt by Obama to borrow a better-delivered line from Elizabeth Warren. Oh, and Joe Biden plagiarized a speech by a British politician and friend named Neil Kinnock. So, really, who cares except professors like me?
Somebody should do a history of plagiarism in presidential contests. Just don't ask Doris Kearns Goodwin to write it. You may not remember this shit, but I do.