Obama spoke at a rally at the University of Southern California at 2 p.m. Both Boxer and Brown were at the rally.
It was estimated about 37,500 people attended the rally, according to USC.
The rally was the fifth in a series of what the Democratic National Committee has dubbed "Moving America Forward," where Obama has stressed the need for young voters to be engaged in the political process.
Before they came to the rally, Obama and Boxer attended a private fundraiser with more than 300 Democrats. Boxer will get about $250,000.
"The other side, they want to take us back. They want to take us back, back to the Bush policies. They did not work, did they?" said Boxer at USC.
"Just like you did in 2008, you can defy the conventional wisdom, the conventional wisdom that says young people are apathetic, the conventional wisdom that says you can't beat the cynicism in politics," said Obama.
Obama paid special attention to the Republican leadership, which he accused of wanting to relive the past.
"'So maybe if we just sit on the sidelines, say no to everything, and then point our fingers at Obama and say he's to blame,' they figure that maybe you all would forget that they caused the mess in the first place," said Obama.
Obama left USC to go to Spanish-language Univision in Glendale. The president taped an interview with popular radio host Eddie "Piolin" Sotelo. The Piolin show gets a large Latino audience and Obama has given interviews to the host in the past.
Obama left L.A. for Las Vegas to campaign for Senator Harry Reid. He's in a tough battle as Senate majority leader to retain his seat.