Former first lady Michelle Obama Michelle LeVaughn Robinson ObamaBlack stars reimagine 'Friends' to get out the vote Obama shares phone number to find out how Americans are planning to vote Michelle Obama: 'Don't listen to people who will say that somehow voting is rigged' MORE has slammed President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE as she begins touring for her memoir, but Trump has not hit back in earnest — yet.

Observers across the political spectrum say that may be because Trump, who normally returns every jab thrown his way, realizes the dangers of getting into a spat with Obama, who enjoys historic status as the nation’s only black first lady and is broadly popular.

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“Her voice carries moral authority at a time when people are very hungry for public figures who take a strong moral stance,” said Katherine Jellison, a history professor at Ohio University and an expert on first ladies. “Even Donald Trump is hands-off when it comes to Michelle Obama.”

Some Republicans are hoping the president keeps up his uncommon show of restraint.

“There is no advantage in a sitting president criticizing a former first lady,” said GOP strategist Matt Mackowiak. “If I was offering strategic advice, I would encourage the White House to resist a response.”

Obama, who was often ambivalent about her husband’s political ambitions and the scrutiny that it brought on her and the couple’s two children, has become more comfortable in the spotlight over time.

She stepped back onto center stage with the publication of her memoir, “Becoming,” on Tuesday. The book immediately shot to the top of best-seller lists.

Obama levels a number of criticisms against Trump in “Becoming,” even though politics is not the main thrust of the book.

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There has never been any secret about Obama's dislike of Trump and his policies. But she makes those points more explicitly than ever in the memoir and in the interviews she has been conducting to publicize it.

She refers to him in the book as a “a bully” who has been “challenging the dignity of our country with practically his every utterance."

Obama also writes that she will “never forgive” Trump for his actions in promoting the false “birther” theory suggesting that former President Obama had not been born in the United States and was therefore ineligible to be commander in chief.

The former first lady says Trump’s words and actions were "putting my family’s safety at risk” at that time.

Trump has long been uncharacteristically reluctant to hit back at Michelle Obama. She aimed barbs at him a number of times during the 2016 campaign, though often without mentioning him by name.

The two most prominent examples came in her “we go high” speech at that year’s Democratic National Convention, and in a later address in New Hampshire, after the release of the “Access Hollywood” tape that showed Trump boasting about grabbing women by the genitals.

"Now is the time for all of us to stand up and say enough is enough. This has got to stop right now," Obama said after the tape became public.

Those attacks did not stop Trump from getting elected. Obama writes in “Becoming” that she went to bed on the night of the 2016 election before Trump’s win was confirmed because “I wanted to not know that fact for as long as I possibly could.”

An online archive of Trump’s tweets indicates that he has never tweeted about the former first lady. Asked earlier this week about her comment regarding never forgiving him, Trump was unusually mild.

“She got paid a lot of money to write a book and they always expect a little controversy,” the president said, before pivoting to criticism of former President Obama.

Karine Jean-Pierre, a national spokeswoman and senior advisor to progressive group MoveOn, said that “even Donald Trump, when he was asked directly about Michelle Obama, he doubled down on Barack Obama Barack Hussein ObamaThe Memo: Trump's strengths complicate election picture Obama shares phone number to find out how Americans are planning to vote Democrats' troubling adventure in a 'Wonderland' without 'rule of law' MORE. I think even he realizes he can’t play on the same playing field as her.”

Jean-Pierre also noted that Michelle Obama had been tempered by facing previous harsh criticism.

During her husband’s initial run for the White House in 2008, for example, Obama was blasted for some of her remarks on the campaign trail, notably when she said that “for the first time in my adult life, I am really proud of my country.”

To conservative critics, the comment was seen as both narcissistic and lacking in patriotism.

In turn, Obama’s defenders saw the negative reaction as carrying a thinly-disguised racial subtext. In one infamous example, the conservative National Review magazine put an unflattering photo of Obama on its cover with the headline “Mrs. Grievance.”

“There was a disgusting campaign to portray Michelle Obama as the angry black woman using every stereotype under the sun,” Jean-Pierre said.

Republicans such as Mackowiak also note that there are dangers in Trump directly attacking the former first lady, 10 years on from those controversies.

“There are always dangers when a man in public life attacks a woman, and those dangers are more pronounced when a white man attacks an African-American woman,” he said.

Those dynamics have not stopped Trump before, however. In recent days, he has hit out at a number of black female journalists: NPR's Yamiche Alcindor, CNN's Abby Phillip and American Urban Radio Networks' April Ryan, who is also a CNN contributor.

Still, Mackowiak added that criticisms from Trump would also doubtless fuel sales of “Becoming.”

“When he responds to something, he elevates it,” he said. “If he responds to her book, he elevates her book — and I doubt he wants her to sell more copies.”

But predicting Trump’s reactions to criticism has always been a dangerous game.

Right now, Obama is making the promotional rounds, giving television interviews in which she often amplifies her criticisms or makes her disdain for her husband’s successor plain.

Asked by Robin Roberts of ABC News on Tuesday who, beyond Hillary Clinton Hillary Diane Rodham ClintonDemocratic groups using Bloomberg money to launch M in Spanish language ads in Florida The Hill's Campaign Report: Presidential polls tighten weeks out from Election Day More than 50 Latino faith leaders endorse Biden MORE, she considered capable of taking on Trump in 2020, Obama responded pointedly: "I think, at this point, everybody's qualified and everyone should run. I might even tap Sasha” — the Obamas’ younger daughter, who is 17.

Given remarks like that, betting on Trump’s continued silence is a risky endeavor — even when letting loose would hold such obvious perils for the president.

The Memo is a reported column by Niall Stanage, focused primarily on Donald Trump’s presidency.