He never mentioned the name of his chief opponent, Democratic US Sen. Dianne Feinstein. Instead he offered implicit criticisms of her leadership, charging that it was no longer time to be polite or patient -- a reference to her comments earlier this year that voters should be patient with the Trump presidency.
"There is no place for capitulation. Never," said de León, a Democrat who is California's Senate President pro Tempore. He argued the state could no longer "play at the margins."
"The DC playbook is obsolete," de León said. "It's time for the people of California to bring the agenda to Washington, not the other way around."
De León, who was introduced by a young undocumented immigrant from Guatemala, touted his work on immigration and climate change issues. He described himself as a product of the California Dream -- the son of a single mother with a third-grade education and someone who intimately knows what it's like to live below the poverty line.
"I know -- firsthand -- that California works best when everyone has the opportunity to succeed. It's a lesson that's hard to learn from the back of a limousine, or behind gated walls," he said in a reference to Feinstein's wealth. "And it's a lesson that guided me every day as the president of the California State Senate -- the most productive and progressive policymaking body in America."
He noted that powerful interests within the party have criticized his decision to run against the veteran senator, who has wrapped up the endorsements of most of the top Democratic leaders in the state, including US Sen. Kamala Harris.
"You know, my whole life I've been told to 'wait my turn' and 'know my place,'" he said. "Well, it's California's turn to lead -- and California's 'place' is to be a shining example for the world -- and a stark contrast to the failures of Washington."
He said California would lead "the resistance" to Trump. "Now is the time," he said, for a senator who leads "not from the sideline, but loud and proud from the front line."
In repeated criticisms of "Washington," he noted that California has passed its own immigrant rights and protections. He touted his bill, which won national attention, making California a "sanctuary state."
De León acknowledged that fundraising would be a challenge, particularly when facing an opponent with the ability to finance her own campaign. Feinstein had $3.6 million in cash on hand in her campaign account earlier this summer. De León cannot transfer any of the money he raised for his state campaigns; he must start from scratch with his federal account.
"I do not have the ability to pick up a pen and write myself a personal check, so there is a clear contrast there," he said during a gaggle after his speech. "That is the contrast in this campaign (between) someone who is a billionaire to someone who grew up in the humblest neighborhoods in California."
Many of the voters filling the seats at the launch of de León's campaign were members of the state's labor unions. Members of the SEIU United Workers West, many of them janitors, arrived in a parade led by men and women playing drums.
Labor is likely to be bitterly divided in California's top-two primary, which will include candidates of all political parties.
The United Farm Workers endorsed Feinstein shortly after she announced her re-election campaign, a testament to her work on California's water scarcity problems in the Central Valley. In a statement, UFW president Arturo S. Rodriguez said the group endorsed Feinstein because during a time when farm workers and immigrants are under attack, "now more than ever we need a genuine champion with great experience who commands respect and authority in the United States Senate."
"After more than a decade of first-hand experience working directly with her, farm workers have discovered Dianne Feinstein has consistently been a tried and true champion," Rodriguez said. "She is tough and sticks with you through thick and thin. Even when we have disagreed with her, we have trusted that she will strive for compromise and solutions."
Bill Carrick, Feinstein's campaign consultant, was not immediately available for comment. But he has told CNN over the past week that de León is a "term-limited guy looking for a gig."