Hillary Clinton, the 2016 Democratic presidential nominee and the person with the most experience competing and debating against President Trump, will throw her support behind Joseph R. Biden Jr. on Tuesday, the latest party leader to try to make the case for returning the White House to Democratic hands in November.

Mrs. Clinton is expected to announce her endorsement on Tuesday afternoon, according to a person familiar with the plans, and appear with Mr. Biden in an online conversation about the impact of the coronavirus on women.

On her Twitter account Tuesday, Mrs. Clinton all but confirmed the endorsement as she disclosed that she will be appearing at the online event with Mr. Biden.