Kevin de León, president of the California Senate, says he will challenge U.S. Sen. Dianne Feinstein in next year's election. Both are Democrats.
De León announced via email Sunday that he's ready to run on his progressive record fighting climate change and pushing for immigration reform, women's rights and quality education.
His announcement did not mention Feinstein by name. She announced last week that that she'll run for her fifth full term. Senators serve six-year terms.
De León, 50, said he wants to join the U.S. Senate to fight the policies of President Donald Trump.
"He disregards our voices. Demonizes our diversity. Attacks our civil rights, our clean air, our health access and our public safety," the campaign email said about the president.
De León will make a formal public announcement in Los Angeles on Wednesday.
He was elected to the state Senate in 2010 and became its president pro tem in 2014. De León previously served in the California Assembly.
Feinstein, who has been in the Senate since 1992, is the ranking Democrat on the Senate's Judiciary Committee.
She has never faced a serious electoral challenge from within her party but her popularity isn't as high as it once was. This election has also highlighted a serious rift among Democrats about which way the party should go in the wake of Donal Trump's presidential victory.
was endorsed by fellow Democrats including California's other U.S. senator, Kamala Harris, and Los Angeles Mayor Eric Garcetti.
"Lots more to do: ending gun violence, combating climate change, access to health care," Feinstein said on Twitter.
Feinstein has focused, often in futility, on gun control issues and immigration. She is also a senior member of the powerful Appropriations Committee, a post that has enabled her to tend to California's needs, and the intelligence panel.
She is an environmentalist but worked with House Majority Leader Kevin McCarthy, a California Republican, to broker an agreement on divisive Central Valley water issues that was opposed by former Democratic Sen. Barbara Boxer.
Feinstein, who had a pacemaker implanted in January, is the oldest senator in a chamber where it's not uncommon for people to serve into their 80s.
Will voters favor Feinstein's experience or will they prefer de León's message of change?