"It's always better to be candid than coy. While I am humbled by the widespread encouragement of so many and hold in the highest esteem those who serve us in federal office, I know that my head and my heart, my young family's future, and our unfinished work all remain firmly in the State of California --- not Washington D.C," he wrote
"Therefore I will not seek election to the U.S. Senate in 2016."
His decision not to run focuses attention on California Attorney General Kamala Harris, another California rising star who's considered a top contender for the seat if she jumps in.
Both have been developing their statewide and national profiles, and they also share the same California-based strategists, increasing the likelihood that if one runs for Senate, the other would opt out.
California political observers speculated before Newsom's announcement that they two might have struck a pact where Harris runs for Boxer's seat and Newsom waits to run for Governor after his two terms as lieutenant governor.
She was mum on the topic when asked about her future political plans by the Los Angeles Times
"I am just enjoying this day, thank you," she said.
But she won't have a walk to the seat if she runs. A wide array of potential contenders
in both parties have expressed interest in the seat, including billionaire environmentalist Tom Steyer and, over the weekend, former Los Angeles Mayor Antonio Villaraigosa, another of the state's rising Democratic stars.
California's jungle primary system means all candidates, regardless of party, face off on the primary ballot, and the top-two vote-getters head to a runoff in November.
The prospect has some Republicans hopeful that too many Democratic candidates running could split the party's vote and leave them an opportunity to make it through to the second round of voting. They admit, however, that's a long-shot — the GOP's bench in California remains thin, and the state is reliably blue.