Former US President Barack Obama and his wife Michelle have endorsed Vice President Kamala Harris as their choice for president. Harris' campaign team released a video on Friday showing the moment the Obamas reached out to her on the campaign trail.

Barack Obama said he and Michelle could not "be prouder" to endorse her and to do everything they can to get her to the Oval Office. Michelle Obama said she feels the election is going to be "historic" and that they will all "roll up their sleeves" to make sure Harris wins.

Harris responded by saying their support "means so much" to her.

The Obamas continue to be highly popular with Democratic supporters and are prodigious fundraising draws. Their endorsement may help spur momentum for Harris, with the Democratic National Committee preparing for a vote to name its official candidate.

Meanwhile, the official candidate for vice president on the Republican ticket is fighting what could become a setback for his campaign.

In an interview three years ago, JD Vance described some Democratic lawmakers as "childless cat ladies." He said Harris and others who do not have biological children do not have a "direct stake" in America's future.

Harris is stepmother to her husband's children from a previous marriage. Her stepdaughter and others, including the actor Jennifer Aniston, have criticized the comment.

US media say some Republicans now worry that Vance might have been their "worst choice" for vice president and could hurt their chances of winning in November.

Vance has defended himself, saying his comment is not a criticism against people who do not have children. He said he was taking aim at the Democrats for what he calls their "anti-family" and "anti-child" policies