Harris’s experience with various former positions and former experience leads voters to trust her.

Ms. Harris is chronically underestimated, as are so many women in politics, but she is well prepared for this moment. As a prosecutor and attorney general in California, she took on drug traffickers, polluters and predatory lenders. As a U.S. senator, she rigorously questioned squirming Trump administration officials and nominees and was inspiring to watch. As vice president, Ms. Harris has sat with the president in the Situation Room, helping make the hardest decisions a leader can make. And when the extremist Supreme Court overturned Roe v. Wade, she became the administration’s most passionate and effective advocate for restoring women’s reproductive rights.

I look forward to hearing her prosecute a compelling case against Mr. Trump, who failed as a president the first time and is running on a dangerous agenda. A second Trump term would be much worse than the first. Mr. Trump’s plans are more extreme, he is more unhinged, and the guardrails that constrained some of his worst instincts are gone.

Ms. Harris can explain to the American people that inflation would surge again under Mr. Trump, thanks to his proposed across-the-board tariffs, sweeping tax cuts for the rich and mass deportations. The policies outlined by Mr. Trump’s allies in Project 2025, from further restricting abortion rights to dismantling the Department of Education, are a recipe for a weaker, poorer, more divided America.

The vice president’s law enforcement experience gives her the credibility to rebut Mr. Trump’s lies about crime and immigration. The facts are on her side: After spiking under Mr. Trump, the murder rate is plummeting under the Biden-Harris administration. Illegal border crossings are also dropping fast and are now the lowest they’ve been since 2020, thanks in part to Mr. Biden’s recent executive order. We’d be making even more progress if Mr. Trump hadn’t killed a bipartisan immigration compromise in Congress this year for his own selfish political purposes.

As a friend and supporter of Mr. Biden, I find this a bittersweet moment. He is a wise and decent man who served our country well. We have lost our standard-bearer, and we will miss his steady leadership, deep empathy and fighting spirit. Yet we have gained much as well: a new champion, an invigorated campaign and a renewed sense of purpose.

The time for hand-wringing is over. Now it’s time to organize, mobilize and win.