US President Joe Biden has held his first campaign rally since he announced in April that he will seek a second term in the 2024 presidential election.

About 2,000 labor union members gathered for Saturday's event in Pennsylvania, where Biden was born.

Biden said he is "proud to be the most pro-union president in American history."

He said, "It's about building an economy literally, not figuratively, from the bottom up and the middle out, not the top down." He added, "Because when the middle class does well, everybody does well."

Pennsylvania is an important swing state, where workers' votes are said to be key.

Some Americans have concerns about Biden's age as he will turn 81 in November. But there have been no moves within the Democratic Party so far to nominate other candidates.

The Republican Party has 11 candidates who are seeking the GOP's presidential nomination. They include former President Donald Trump and Florida Governor Ron DeSantis, who is seen as Trump's biggest rival.