Stung by criticism over her 2016 Republican convention appearance, US First Lady Melania Trump has been a circumspect participant in public efforts to boost her husband's presidency.

But her projection of calm and compassion in discussing Covid-19, racial tensions and opioid abuse during her Tuesday night address won widespread plaudits.

While Donald Trump's wife expressed obligatory support for his re-election bid, the 50-year-old conveyed something the president and his inner circle rarely do: a recognition of Americans suffering from a pandemic that has upended millions of lives.

"My deepest sympathy goes out to everyone who has lost a loved one and my prayers are with those who are ill or suffering," she said in a highly anticipated White House speech.

"I know many people are anxious and some feel helpless. I want you to know you are not alone."

Melania's timbre and messaging contrasted to the raw, aggressive politics of Trump's children who spoke before her.

Also absent was her husband's boastful claims about responding to the pandemic, his dismissive tone to critics, and astringent "law and order" threats to protesters including Black Lives Matter supporters.

Instead the Slovenian-born former model deployed a warm personal touch seemingly aimed at softening the edges of a contentious president.

She shared her own immigration story of moving to the United States in the 1990s and becoming a citizen, proud at being "able to achieve my own American dream."

And she even uttered a word seldom mentioned by Trump or any White House staffer: "mistakes."

"Like all of you, I have reflected on the racial unrest in our country. It is a harsh reality that we are not proud of parts of our history," Melania Trump said. "Instead of tearing things down, let's reflect on our mistakes."

The address marked an improvement from her 2016 convention speech, when she was found to have lifted some wording from an earlier address by her predecessor Michelle Obama. 

Melania pulled off a tricky balancing act: connect with voters and appeal for four more years of a Trump presidency without ranting against the opposition, something she said would only serve to "divide the country further."

"It was head-spinning to hear (Melania Trump) acknowledge the devastation of COVID-19; racial injustice... admit mistakes and hear others," veteran political analyst David Axelrod said on Twitter, adding it was "completely at odds with her husband's approach."

White House aides told US media that her address was her own words, delivered without the help of professional speechwriters.

'Empathy and sympathy' 

In contrast, the president's son Eric Trump tore into Democrats earlier in the evening, saying a Joe Biden administration would seek to "burn the Stars and Stripes that represent patriotism and the American dream."

"I think she did what she had to do," Katherine Jellison, a history professor at Ohio University, told AFP.

"While the rest of the (convention) participants presented language that emphasized anger and fear, she showed empathy and sympathy for a country suffering two major crises -- Covid-19 and a reckoning with the nation's racial sins."

Melania Trump has promoted anti-bully work as part of her "Be Best" campaign but she has largely remained in the background the past four years.

On Tuesday she made a grand entrance, striding through the colonnade and into the Rose Garden, which she recently redesigned, to address rows of mostly maskless guests, including the president.

Clad in an olive green suit with boxy shoulders and wide lapels, she gave the appearance of a fashionable militant on a mission to persuade American hearts and minds.

But will she be able to marshal enough new recruits to help Trump defeat Democrat Biden, who leads in the polls, in November?

"I think she walked the tightrope pretty well and perhaps did pull in a few voters -- particularly some white women -- from the margins," said Jellison, adding though that Melania would likely not be seen as a truly unifying force.

"Anyone associated with the Trump presidency ultimately emerges as a polarizing figure."