At a rally for Democratic presidential nominee Hillary Clinton in Philadelphia Sept. 28, first lady Michelle Obama attacked Clinton's opponent for his temper, his remarks about women and his questions about her husband's birth certificate. (Sarah Parnass/The Washington Post)

Ever since her much-lauded speech at the Democratic National Convention, I've been waiting to see when and where first lady Michelle Obama would make her presence — and her popularity — known in the fall presidential campaign.

Today looks to be that day. Obama was campaigning in Philadelphia — at La Salle University, to be exact — when she absolutely let loose on Republican nominee Donald Trump and his long-running birther insinuations about her husband.

Just in case that's too small for you to read, there are two key passages in Obama's remarks above. (You can watch her speech here.)

The first deals with Trump's five-year quest to prove that President Obama was not born in the United States, a quest he kind-of-sort-of abandoned last week with a single-sentence declaration of the president's citizenship at the end of an event with military veterans. Michelle Obama described Trump's birther campaign this way: "Hurtful, deceitful questions deliberately designed to undermine his presidency, questions that cannot be blamed on others or swept under the rug by an insincere sentence uttered at a press conference."

The other covers her assessment of Trump's words and deeds — and what they tell us about the sort of president he would be. "If a candidate is erratic and threatening, if a candidate traffics in prejudice, fears and lies on the campaign trail, if a candidate thinks that not paying taxes makes you smart, or that it's good business when people lose their homes. If a candidate regularly and flippantly makes cruel and insulting comments about women, about how we look and how we act, well, sadly, that's who that candidate really is."

There's not a lot of holding back there on the part of the first lady. That is an unvarnished stripping of the bark off Trump in a way her husband — by dint of the fact he is still the sitting president of the United States — simply cannot do.

That Michelle Obama said it is interesting. Where she said it is even more telling.

This is Philadelphia and the Philadelphia media market. Yes, it is home to a large African American community but Obama's words will also be heard — whether in person or on the local news — by suburban white women in Montgomery, Delaware and Chester counties. Michelle Obama is a hugely popular figure, generally speaking (in an August Gallup poll, 64 percent of Americans had a favorable opinion of her), but she is particularly popular among women. In a post-Democratic convention Fox News Channel poll, 54 percent of women said they felt strongly favorable toward Obama, with an additional 13 percent saying they felt somewhat favorable.

Between Trump's string of negative comments about women — comments the Clinton campaign has documented in a series of TV ads — and his current fight with former Miss Universe Alicia Machado, Michelle Obama's harsh words for the Republican nominee could take an already difficult situation for him and make it that much worse.

The decision to deploy her at this exact moment — and for her to go after Trump so hard in this speech — speaks to the careful strategy of the Clinton campaign as it seeks to make the case to women that Trump is totally unacceptable. If Clinton wins, she'll owe Michelle Obama a very big "thank you."