LOS ANGELES — It is no easy task to upstage Oprah Winfrey, Caroline Kennedy, and Stevie Wonder at the same time, but the first lady of California, Maria Shriver, managed to do just that with her surprise endorsement of Senator Obama of Illinois two days before his crucial Super Tuesday showdown with Senator Clinton.

“If Barack Obama was a state, he’d be California,” Ms. Shriver told a rally for Mr. Obama’s presidential campaign here. “I mean, think about it: diverse, open, smart, independent, bucks tradition, innovative, inspiring, dreamer, leader.”

Ms. Shriver told the crowd that her decision to drop in on the high-profile rally at the University of California at Los Angeles was a spur of the moment one. As evidence, the former television broadcaster noted that her hair was not combed, that she was not wearing makeup, and that she was attired in an outfit she wore to her daughter’s horse riding practice earlier in the day.

Ms. Shriver said she had gotten conflicting advice about whether she should make an endorsement, but ultimately decided she needed to speak out. She went on to deliver a forceful call to Californians to swing the Democratic presidential race in Mr. Obama’s favor.

“Elections are a lot like life, they are made up of moments,” Ms. Shriver said. “This election is about a moment. This is a moment not just for the United States of America, this is a moment not just for the Democratic Party, this is also a moment for California. … We’re at the epicenter of change,” she said, adding, “We can lead this country. So goes California, so goes the nation.”

News coverage of Ms. Shriver’s endorsement is likely to reverberate through election day, in part because her husband, Governor Schwarzenegger, has endorsed Senator McCain and reporters are likely to question the governor about the political rift in his household.

The headliner of yesterday’s rally, Ms. Winfrey, was marking her first return to the campaign trail since she stumped for Mr. Obama in New Hampshire, Iowa, and South Carolina in December. Diving into the thorny issues of race and gender that have preoccupied the Democratic race, the talk show host and entrepreneur told the crowd that not all the feedback she got from those early appearances was positive.

“After Iowa, some woman had the nerve to say to me, ‘How could you, Oprah? How could you? You’re a traitor to your gender,'” Ms. Winfrey recalled. “I was both surprised by that comment and insulted because I’ve been a woman my whole life and every part of me believes in the empowerment of women but the truth is I’m a free woman. … Being free means you get to think for yourself.

“So I say, I am not a traitor,” Ms. Winfrey declared. “I’m just following my own truth and that truth has led me to Barack Obama.”

The daytime television legend said she was also offended by people who suggested she was supporting Mr. Obama solely because of his race. “Don’t play me small. I’m not that small. … I would never vote for anybody because of gender or race. I’m not voting for Barack Obama because he’s black. I’m voting for Barack Obama because he’s brilliant,” she declared, prompting a sustained roar from the crowd.

Perhaps because of the Super Bowl, the crowd for yesterday’s rally was a modest one compared with the outsize audiences the leading Democratic campaigns have been drawing nationwide. The UCLA arena, which seats about 13,000, was less than half full. By comparison, Mr. Obama drew about 15,000 people last week for a rally in Boise, Idaho, and President Clinton pulled in about 11,000 in Davis, Calif., last month for a speech on his wife’s behalf.

While some have speculated that Ms. Winfrey’s involvement might draw fans who want to see her but do not give a whit about Mr. Obama, the opposite seemed to be true yesterday. For the in-person crowd, the gathering was more about motivation than conversion, as most arrived already committed to Mr. Obama. Women outnumbered men, but not by as much as might be expected for an event designed to attract female voters. Many attendees said they were impressed with Ms. Shriver’s message about a “moment” for change and with another impassioned speech yesterday in which Mr. Obama’s wife, Michelle, argued that inspiration and hope are critical to delivering important change in society.

“I’ve never been to a political rally before,” Claudine Harris, 29, a county public health investigator, said. “I’m excited. I’m pumped up.”

A teacher from Long Beach, Calif., Lakeba Fondren, 39, said Ms. Obama’s message about the practical value of hope is crucial to schoolchildren. “It’s very important what she was saying, for students and teachers,” Ms. Fondren said.

The rally wrapped up in time for most attendees to get home in time to catch most of the Super Bowl, but Ms. Fondren said she would skip it to make phone calls for the campaign. “I’m not going to the Super Bowl, I’m so fired up,” she said.