Sen. Kirsten Gillibrand of New York left it at that.
Sen. Cory Booker of New Jersey went further, directing his 1.5 million followers to a website where they could donate money to Harris.
And Sen. Elizabeth Warren of Massachusetts, the biggest catch of the day, offered a full-scale endorsement, along with a plea for donations to help Harris “stand up to the powerful interests and win."
Warren, whose crusade against banking abuses has made her one of the nation’s most popular figures among liberals, called Harris “a smart, tough, and experienced prosecutor who has consistently stood up to Wall Street.”
Together, the three senators rallying behind their fellow Democrat gave Harris a chance to try to establish herself as an early front-runner in the Senate contest. Many potential rivals are still weighing whether to run in the June 2016 primary.
The displays of East Coast support for Harris reflect years of effort to build a national political network.
But one of her most formidable potential Democratic opponents, former Los Angeles Mayor Antonio Villaraigosa, has worked even longer to establish a place for himself in national politics.
Since Boxer announced a week ago that she would not seek reelection, Villaraigosa has been seeking advice from veteran strategists and other key figures in California politics.
Democratic consultant Garry South, who visited the former mayor at his Santa Monica home to discuss the Senate race Monday, said Villaraigosa seemed “very likely” to run for Boxer’s seat.
“I think his calculations are correct that this is sort of an unexpected opportunity,” said South, who is not working for the former mayor.
Villaraigosa’s political base in Southern California, particularly among Latinos, could offset the Bay Area stronghold of Harris, a former San Francisco district attorney.
Villaraigosa's national political connections would also help him raise money, an extremely time-consuming task because of strict federal donation limits and the high cost of running a campaign in California. Villaraigosa was chairman of the 2012 Democratic National Convention. He is also a former president of the U.S. Conference of Mayors.
Villaraigosa’s longstanding ties with the Clinton family could also prove helpful in a presidential election year when Hillary Clinton is the most likely Democratic nominee.
Villaraigosa was a national co-chairman of Hillary Rodham Clinton’s 2008 presidential campaign. Harris was co-chair of Obama’s 2008 campaign in California, a role unlikely to be forgotten by the Clintons.
Harris might also be facing the financial juggernaut of another potential Democratic rival in the Senate race: Bay Area billionaire Tom Steyer, a former hedge fund chief. Steyer, who spent tens of millions of dollars backing Democratic candidates last year as part of his efforts against global warming, has said he might run for Boxer’s seat.
Also in the mix could be state Treasurer John Chiang, some members of the U.S. House of Representatives, two former chairmen of the state Republican Party and a host of other elected officials.