From the beginning of President Biden’s ill-fated re-election bid, Democrats struggled to frame the race as a choice between two radically different visions, rather than a referendum on Mr. Biden’s age and abilities.

But now that he has dropped out and Vice President Kamala Harris is cruising toward the Democratic nomination, her party sees fresh opportunities to turn the public’s attention back to the vulnerabilities of former President Donald J. Trump on key issues for voters, including abortion rights, fundamental democratic principles and questions of economic fairness.

Democrats hope that Ms. Harris — a history-making former prosecutor who is nearly two decades younger than Mr. Trump — can draw a sharp new set of contrasts and tap into arguments that were out of reach for Mr. Biden, 81, starting with the issue of age but extending into matters of policy and personality.

That is especially clear on abortion rights. Mr. Biden, a practicing Catholic, has long been uncomfortable discussing the issue, or even saying the word “abortion.” Ms. Harris, by contrast, speaks easily and openly about reproductive health and has held campaign events alongside women sharing their stories of miscarriages, abortions and challenges with fertility.

Republicans, for their part, plan to argue that Ms. Harris still owns the unpopular parts of Mr. Biden’s record, saying that she is also weak on issues like inflation and immigration. Polling has shown Mr. Trump with significant advantages on many of the most important issues for voters, other than abortion rights.

But in interviews, Democrats were eager to make the race about character and experience, noting Ms. Harris’s law enforcement background and Mr. Trump’s litany of legal problems.