WASHINGTON (BLOOMBERG) - Democratic presidential nominee Joe Biden is responding to US President Donald Trump's campaign ads designed to inspire fear of protesters with a series of advertisements in key swing states that shows positive messages of unity.

"I've been looking to lower the temperature in this country, not raise it," Mr Biden says in one 60-second ad, running largely on morning news and daytime television.

It is targeted at metro areas in the key battleground states of Michigan, Pennsylvania and North Carolina.

His language suggests a different strategy than Democrat Hillary Clinton employed in the 2016 campaign, seeking to offer voters a reason to vote for him and not just against Mr Trump. At that time, it was Mrs Clinton on the attack and Mr Trump running a more positive campaign.

"I think it's no accident," said Ms Erika Franklin Fowler, director of the Wesleyan Media Project, which tracks political ads. "They are intended to convey a presidential tone of calm and restoration of order. That's very much the strategy there, in contrast to Trump, who is very loud and very negative."

Mr Biden's new ads embrace the Black Lives Matter movement, call for police reform and blame Mr Trump for the rioting and looting going on in some US cities.

And they are also markedly different in tenor than Mr Trump's most recent ads, forsaking the narration and ominous music characteristic of attack ads.

Another ad launched this week features Mr Biden and his running mate, Senator Kamala Harris, talking about police accountability.

And a third ad running in Florida features the voice of Ms Adrianna Williams, a Black Lives Matter protester from Tallahassee.

"Electing Joe Biden is essential to addressing these issues and getting meaningful change. We have to continue the activism. We have to continue protesting, but we also have to go out and vote," she says.

Since May, Mr Biden's campaign has spent US$36.5 million (S$50 million) on ads that present a positive message and US$5.7 million on negative spots, which criticise or attack an opponent, according to data Bloomberg analysed from the ad tracking firm Advertising Analytics. Another US$25.1 million in ads contain both positive and negative elements.

Mr Trump's ads over the same time period are more often negative.

His campaign has spent an estimated US$50.6 million on ads attacking Mr Biden and the Democrats, and US$7.2 million on spots that present him in a favourable light. He has spent another US$6.4 million on contrast ads.

A pair of Trump ads running in Minnesota and Wisconsin features videos of rioting and burning cars and accuses Mr Biden of "taking a knee" in solidarity with the protests.

But recent polls in key battleground states suggest the ads might help Mr Biden retain his lead over Mr Trump on the issue.

A Marquette University Law School poll released on Wednesday shows Mr Biden with a four-point lead overMr Trump, essentially the same since before the protests that erupted over the police shooting ofMr Jacob Blake in Kenosha, Wisconsin, on Aug 23.

The same poll found 36 per cent of Wisconsin voters approve of Mr Trump's handling of the protests, while 54 per cent disagree.

The 60-second ad that allows Mr Biden to speak in his own words is a tactic the campaign has used before and is likely to do again.

"It gives us an opportunity to tell a fuller story. In battleground states where people are inundated with ads, it helps to break through the noise," said Biden spokesman Mike Gwin.

"I think there's a clear contrast between what we're doing and the advertising you're seeing from Trump," Mr Gwin added.

The format allows the campaign to put Biden into presidential-like settings, drawing a sometimes unspoken contrast between Biden's more measured language and Trump's often unscripted and provocative remarks.

Trump campaign deputy press secretary Samantha Zager said Mr Biden's ads "don't change the fact that Americans have watched Biden's utter lack of courage to stand up to his radical supporters while President Trump has condemned the violence, demanded law and order, and taken action to protect our communities without hesitation".

The most widely aired ad is basically an excerpt from a speech Mr Biden gave in Pittsburgh last week.

"Fires are burning, and we have a president who fans the flames. He can't stop the violence because for years he's fomented it," he says. "If I'm your president, my language would be less divisive."

The ad ends with Mr Biden citing a biblical refrain used by Pope John Paul II: "Be not afraid."