U.S. President Joe Biden said in comments broadcast on Sunday that he would campaign for Democratic candidate Kamala Harris in Pennsylvania, a key battleground state in November's presidential election.

Biden made his remarks in an interview with CBS, his first since announcing in late July that he would pull out of the race against Republican candidate Donald Trump over concerns about the future of U.S. democracy.

Harris leads Trump by four points each in Pennsylvania and two other key swing states, Wisconsin and Michigan, according to polls by the New York Times and Siena College released on Saturday.

Asked whether he would be campaigning for Harris, Biden replied: "Yes."

Biden said he and Pennsylvania Governor Josh Shapiro "are putting together a campaign tour in Pennsylvania. I'm going to be campaigning in other states as well and I'm going to do whatever Kamala thinks I can do to help most."

Further details were not immediately available.

Biden and Harris are due to appear together in Maryland on Thursday to "discuss the progress they are making to lower costs for the American people," the White House said last week.

Harris, who officially became the Democratic Party's presidential nominee last week, on Saturday said she planned to unveil her policy positions in the coming week.

"It'll be focused on the economy and what we need to do to bring down costs, and also strengthen the economy overall," she told reporters.

Harris announced last week that Minnesota Governor Tim Walz would be her vice presidential running mate.

"He's my kind of guy, he's real, he's smart, I've known him for several decades. I think it's a hell of a team," said Biden.

The president, who decided to step down in the wake of a disastrous debate performance against Trump on June 27, said internal polls showed the race would have been very close.

"But what happened was, a number of my Democratic colleagues in the House and Senate thought that I was going to hurt them in the races and I was concerned if I stayed in the race, that would be the topic you'd be interviewing me about," he said.

In an initial excerpt of the interview released last Wednesday, Biden said he was not confident about a peaceful transfer of power in the United States if Trump lost.