The power plant has not produced electricity since January 2012 because of a small radiation leak that caused damage to hundreds of tubes carrying radioactive water.
Southern California Edison asked the Nuclear Regulatory Commission to change the plant's operating rules to permit a single reactor to run at no more than 70 percent power. In a tentative ruling, the NRC staff reviewed a company analysis and agreed that running the reactor at no more than 70 percent power would not significantly increase the risk of any accident.
The issue represents one of a series of regulatory steps Edison faces before being allowed to fire up the reactor, and the preliminary ruling is a small victory for Edison.
Edison has been pushing for a restart of the plant by June. They say the Unit 2 reactor is safe to run in low power.
Sen. Barbara Boxer called the plan to restart the plant before the investigation is complete "dangerous and premature."
"It makes absolutely no sense to even consider taking any steps to reopen San Onofre until these investigations look at every aspect of reopening the plant," Boxer said in a statement.
The agency said it is seeking public comments for 30 days on this proposed ruling, which will be considered in the decision.
The Associated Press contributed to this report.