PHOENIX (AP) — First lady Michelle Obama has emerged as perhaps the most effective Donald Trump critic in the Democrats' lineup, and she's done it without ever uttering two key words: Donald Trump.

In her five speeches while campaigning for Hillary Clinton, the first lady has never said the Republican nominee's name. She's talked about "a candidate" or "this candidate" and dedicated much of her time to a searing indictment of his words and positions. But throughout her buzzworthy takedowns, Trump remains the man who shall remain nameless.

Don't look for Mrs. Obama to start to change course in Phoenix Thursday, when she is scheduled to headline a rally. Her appearance in Arizona is a mission to crack open new territory in a GOP-leaning state polls show is now competitive.

The Clinton campaign and Mrs. Obama's staff are reluctant to discuss motives for the obvious omission. But Mrs. Obama's rhetoric shows her trying to balance her position as first lady — a figure long viewed as out of the political fray — while also holding little back in a race she clearly feels strongly about.

"If a candidate traffics in prejudice ... well, sadly that's how that candidate really is," Mrs. Obama said this month in Pennsylvania. "And trust me, a candidate is not going to suddenly change once they're in office — just the opposite."

Last week, she described Trump as "a candidate for president of the United States who, over the course of his lifetime and the course of this campaign, has said things about women that are so shocking, so demeaning, that I simply will not repeat anything here today."

Political speakers are often coached to avoid using opponents' names or titles, to deny them any measure of extra publicity or credibility.

It's a time-worn demonstration of disdain by denial, said Mary E. Stuckey, a scholar of political oratory at Georgia State University. By marginalizing him personally, Mrs. Obama also aims to marginalize what he stands for as a candidate.

It may just be coincidence, but Mrs. Obama's speech Thursday is at the downtown convention center in Phoenix where Trump issued a reaffirmation of his immigration policy proposals, which Clinton sharply opposes.

"Naming, of course, is a form of power. It defines things and makes them real," Stuckey said. "To refuse to name is also to refuse to recognize something."

But others see additional possible motives in Mrs. Obama's rhetoric.

Where previous first ladies have typically played the role of loyal spouse and burnished their husbands' records while campaigning, Mrs. Obama has taken a different tact, said Anita McBride, who was chief of staff to first lady Laura Bush.

"Her speeches have been more political," McBride said. "Her speeches at the Democratic National Convention and in New Hampshire last week were sharper, more targeted and more cutting than anything I've seen in a previous first lady."

Mrs. Obama spoke at length at the Manchester rally about the release this month of a video from a 2005 "Access Hollywod" interview, where Trump said into a microphone he did know was live, that he used his celebrity to make sexual advances on women without their consent.

In the weeks that followed, nine women have accused Trump over the past 30 years of kissing and groping them against their will.

Mrs. Obama's response was an effort to starkly refer to Trump as "this candidate actually bragging about sexually assaulting women. I can't believe I'm saying that."

Trump has made a habit of retaliating against his critics. The only time he has mentioned Mrs. Obama during the campaign has been to attempt to poke holes in her support for Clinton by reminding voters of the fierce fight for the 2008 Democratic nomination Clinton fought against President Barack Obama.

Clinton's daughter, Chelsea, has not followed Michelle Obama's example. Speaking at Arizona State University Wednesday, she sprinkled Trump's name throughout her 30-minute speech and question-and-answer session with more than 500 supporters on the campus in Tempe.

That leaves some former Obama administration staff and others suggesting that the first lady finds Trump so objectionable that she refuses to utter his name as a way of denying him credibility.

"I wonder in some ways if she finds his politics and rhetoric so distasteful she can't bring herself to say his name," said the former Bush senior aide McBride. "Clearly, there's a great deal of passion in these speeches."