More than 12,000 nurses in Minnesota were scheduled to walk off their jobs on Thursday.
Leaders held a vigil after approving a one-day strike for 14 hospitals in the Minneapolis-St. Paul area in an impasse over staffing levels and pension benefits.
"They've had enough," said Karen Higgins, a Massachusetts nurse. "It's time to say that we're going to do what we have to do to protect our patients."
"We think their only goal at this is to have the largest nurses' strike in history," said Maureen Schriner, spokeswoman for the Twin Cities hospitals.
Dwaine Duckett, vice president of human resources for the University of California system, called the strike threats "part of a national strategy to gain negotiating leverage and demonstrate nationwide power."
The hospitals have been trying to cut costs. Administrators claim nurses' pay and benefits are two of the biggest expenses.
The hospitals said they have hired enough qualified replacement workers to pick up Thursday's slack.