The president's stern words in a state where he remains popular and Boxer won her last re-election race in a rout underscored the perilous political environment confronting all Democrats in this midterm election year - and showed Obama is all too aware of the dangers.
"I don't want anyone here taking this for granted," he said at a reception at the California Science Center, the first of a trio of fundraisers Monday night for Boxer and the Democratic National Committee.
"Unless she's got that support she might not win this thing, and I don't think that's an acceptable outcome. So I want everyone to work hard," the president said.
All incumbents face an uphill battle because of the economy, Obama said, though he insisted it's turning around.
At the second event Obama faced a handful of hecklers demanding to know what he was going to do to get rid of the "don't ask don't tell" policy that prevents gays from serving openly in the military. The heckling grew so insistent that Obama responded, saying that he and Boxer supported overturning the policy.
"So I don't know why you're hollering," Obama said, telling them to yell at people who oppose lifting the ban.
Their shouts were drowned out by cries of "Yes we can! Yes we can!" from others in the crowd.
Obama also used the fundraisers to make a pitch for the financial regulation overhaul legislation awaiting action in the Senate, saying it was necessary to rein in Wall Street and attacking Senate Republican Leader Mitch McConnell by name for opposing the bill. Obama plans a speech on the issue Thursday in New York.
Boxer herself, who's seeking a fourth term, referenced the perceived enthusiasm gap between the Democratic base and the revved-up tea party movement, and said her supporters had to battle back.
"Are you ready to go toe to toe to them, cup by cup by cup? Because I am," Boxer said. "We're in a run for our life. We're all being tested now. The times are testing us."
The events were expected to raise between $3 million and $3.5 million for the senator and the DNC, with ticket prices ranging from $100 for a reception to $17,600 for dinner with the president. Two events were at the California Science Center and the dinner took place at the Natural History Museum of Los Angeles.
They were among Obama's first appearances on behalf of a candidate since his health care overhaul passed Congress last month.
An independent Field Poll last month found Boxer running about even with the three Republicans who are competing for the nomination in the state's June 8 primary; she has only token primary opposition.
Even so, most analysts give Boxer the advantage, especially compared with some other Senate Democratic candidates or seriously endangered incumbents such as moderate Blanche Lincoln in Arkansas. But whereas Obama might not be welcome in conservative Arkansas, he was cheered in California, where he remains popular - far more popular, in fact, than Boxer.
His approval rating stood at 52 percent in a Field Poll last month. By contrast Boxer was viewed favorably by only 38 percent of California voters.
And unlike in other parts of the country, both Obama and Boxer could benefit from passage of the health care law. A Los Angeles Times/University of Southern California poll this month said California voters have a generally positive view of the health care bill. Forty-six percent said they would be more likely to vote for a lawmaker who supported the bill compared with 29 percent who said they would not be.
Boxer's Republican rivals are state Assemblyman Chuck DeVore, businesswoman Carly Fiorina and former Rep. Tom Campbell.
In a conference call with reporters Monday, Fiorina said Obama is on a "rescue mission" to save Boxer when the state's troubled economy needs help.