The union representing California state prison guards filed a formal notice that it will seek to oust Governor Arnold Schwarzenegger, a move his advisers say they are taking seriously.
The California Correctional Peace Officers Association criticised Mr Schwarzenegger for what it called "catastrophic leadership failings and inept management" since he took office in 2003 - after the removal of then-Governor Gray Davis.
When the union announced its plans yesterday, Mr Schwarzenegger said it was a ploy designed to pressure the administration into granting a large pay raise to prison guards. He said he will not be intimidated.
Mr Schwarzenegger's political adviser, Adam Mendelsohn, described the threat as a political stunt that had nothing to do with policy issues.
He noted that the union issued its threat after it failed to persuade state politicians to bypass contract negotiations with the governor's staff.
Mr Mendelsohn also said the move is caught up in the internal politics of the 30,000-member union.
Rank-and-file prison guards are upset that they have not had a new contract in two years, and the union's president, Mike Jimenez, faces an internal challenge to his re-election at a meeting next week in Las Vegas.
Nevertheless, the governor's campaign team is taking the threat seriously, Mr Mendelsohn said.
"This is a union that has a history of throwing a lot of money at political issues," he said during a conference call with reporters. "You can't be naive to think it's not serious."
The prison guards are among the most well-funded and influential lobbying groups in the capital.
Before he was ousted, Mr Davis was widely criticised for accepting 2.6 million US dollars in campaign donations from the union and then giving its members a 37% pay raise. Mr Schwarzenegger has not taken campaign money from the union.