US Democratic presidential nominee Kamala Harris has promised to be 'a president for all Americans,' as she called for support so she can win the upcoming presidential election.

The incumbent vice president gave her speech on Thursday, the last day of the four-day Democratic National Convention in Chicago.

Harris said the upcoming election will be a precious opportunity to chart "a New Way Forward."

Noting that she comes from a middle-class family, Harris said a strong middle class has always been critical to America's success, and building a strong one will be a defining goal of her presidency.

She stressed her intention to implement economic policies that will include tax cuts for the middle class.

Describing her rival, Republican presidential candidate and former President Donald Trump, as "an unserious man," Harris warned, "The consequences of putting Donald Trump back in the White House are extremely serious."

She said, "Consider not only the chaos and calamity when he was in office, but also the gravity of what happened since he lost the last election."

With voting about two-and-a-half months away, her roughly 40-minute-long speech broadcast live nationwide demonstrated her aim of appealing to a wide range of voters, including swing voters.