Senior campaign aides say Shriver's view is crucial on important decisions.
She was deeply involved in the hiring of strategist Mike Murphy, who helped direct John McCain's presidential campaign in 2000, said campaign officials who also say that along with her cousin, Robert F. Kennedy Jr., she helped assemble environmental advisors for the campaign.
Timothy Shriver said his sister's main concern is making sure Schwarzenegger "doesn't get buffeted by changing opinions."
"She understands who he is -- his commitment to social justice, economic discipline, a competitive free market, his interest in education. She wants to make sure what goes out is him," he said.
"I don't think she has veto power, it's not that kind of relationship. But there's nobody on the planet he respects more than her."
But her friends and family point out that part of her value to Schwarzenegger is being the least self-interested person at the table.
"One hundred percent of her agenda is Arnold -- she doesn't want a job in policy," said Bobby Shriver. "You cross Arnold, and Maria will cut your head off. Whether it's agents or campaign managers, if anyone does something to Arnold she doesn't like, watch out."