At an Oct. 13 Hillary Clinton campaign event, first lady Michelle Obama grew emotional while talking about allegations of sexual assault against Republican nominee Donald Trump. (Youtube/Hillary Clinton)

First lady Michelle Obama on Thursday delivered what may be the most impassioned and effective repudiation of Donald Trump in this presidential campaign. And despite speaking immediately afterward at a rally in Florida, the Republican nominee said nothing.

Which is par for the course. He has never hit back at or really attacked the first lady at all, in fact — either because he likes her, doesn't think it's smart politics, or for some other reason.

I wrote about this phenomenon after the Democratic National Convention in July, when the first lady attacked Trump without using his name, and Trump proceeded to praise her speech. In fact, she seems to be about the only person in politics that Trump won't attack.

But why? We'll let you decide. Again, this is from July:

Last week, we found out that Donald Trump's wife, Melania Trump, is apparently a fan of Michelle Obama's. In explaining her inadvertent plagiarizing of the first lady's 2008 convention speech, Melania Trump's speechwriter said, "A person she has always liked is Michelle Obama."

Is it possible that Melania Trump's husband agrees with her?

As I noted last week, it's not that unusual for prospective or actual first ladies to say nice things about one another. There is something of a kinship, it seems, and they don't often get involved in the political sniping.

But here's the thing: Obama has now delivered two speeches that were very tough on Donald Trump — albeit without mentioning him by name in either. There was an early June commencement speech and now one on Monday night at the Democratic National Convention. Trump's response to each one has been to basically ignore it.

And indeed, despite Obama's status as a first lady many conservative Republicans love to hate, Trump doesn't appear to have ever really gone after her for anything. In fact, after Obama's convention speech, Trump actually praised it in an interview with the Hollywood Reporter.

"I thought her delivery was excellent," he said. "I thought she did a very good job. I liked her speech."

That lack of a response is surprising from a guy who is nothing if not fond of doing things to stir up the GOP base. This is also a guy, mind you, who doesn't take incoming political fire without responding in kind. "I'm a counter-puncher," he has said repeatedly during this campaign. It's his explanation for many of his feuds.

It's also not as though he's opposed to engaging in political fights with women. Rosie O'Donnell, Megyn Kelly, Carly Fiorina, Elizabeth Warren — the list goes on and on. And we all know how Trump feels about Obama's husband.

And yet, even as he tweetstormed during the big speeches of the convention Monday night — especially during speeches by Warren, Cory Booker and Bernie Sanders — not a word about the first lady, even as she made a very forceful case against him in no uncertain terms.

"I want someone with the proven strength to persevere," she said. "Someone who knows this job and takes it seriously. Someone who understands that the issues a president faces are not black and white and cannot be boiled down to 140 characters. Because when you have the nuclear codes at your fingertips and the military in your command, you can’t make snap decisions. You can't have a thin skin or a tendency to lash out. You need to be steady, and measured, and well-informed."

It didn't escape notice:

"Donald Trump ranted about all the DNC speakers on Twitter — except one," Business Insider said.

"There’s One Person Donald Trump Did Not Try to Mess With Last Night: Michelle Obama," Slate said.

Some indeed painted it as Trump not wanting to pick a fight with a powerful and eloquent first lady who gave a fantastic speech.

But Trump's feuds are never quite so calculated. If he doesn't worry about "counter-punching" with Pope Francis — the pope! — why would he shy away from doing the same with Obama? This is not a guy who knows how to just let something go or can be prevailed upon to avoid an unhelpful political feud.

There is something about the first lady, it seems, that's different for Trump. And looking back, it's not clear that he has ever really targeted her in a forceful way.

In fact, four years ago at this very juncture in the presidential process, Trump offered similar praise for her Democratic National Convention speech.

The year 2012 was not back when Trump was a Democrat, mind you. This was after he spent years questioning whether Obama's husband was born in the United States and was a legitimate president. His other tweets during that year's Democratic convention weren't nearly so kind.

And yet, praise for Michelle Obama.

Earlier that year, Trump did take a little dig at the first lady.

But that appears to be about the worst thing he has said about her. Other tweets mentioning her include asking what people thought of her haircut.

And he complimented her and her husband on their 20th anniversary in 2012.

Trump has always been a fan of stirring up controversy and appealing to the GOP base's antipathy toward everything Obama — except, apparently, for Michelle Obama.

Not yet, anyway.

Here are six times Republican presidential contender Donald Trump has insulted women, from Rosie O'Donnell to Ted Cruz's wife, Heidi. (Sarah Parnass and Nicki DeMarco/The Washington Post)