After threats at several field offices around the state, Sen. Barack Obama's campaign is asking labor unions to help provide volunteer security at 27 offices between now and Election Day.

The Obama campaign has 81 offices across the state. Of the 17 in Philadelphia, they want security at six, according to an e-mail sent to union leaders.

The e-mail said there have been threats at three Philly offices. That's on top of an evacuation this week at the South Philly office after they received a threatening letter that contained a suspicious substance, which initial tests determined was sugar.

Obama spokesman Sean Smith declined to go into detail on the other threats made so far, but said that some had been just crank calls.

"It isn't unusual for a campaign to get bizarre phone calls and letters," said Smith. "Emotions can run high this time of year."

Smith said getting extra security was a routine practice as the campaign amps up volunteer hours in the final weeks.

"This is standard operating procedure with most campaigns I've worked on," Smith said. "You have people who work very late hours. They're leaving at one, two in the morning. They're senior citizens and young people."

Smith stressed that the campaign wasn't looking for round-the-clock security guards, but for people who would check in on offices a few times a night. He wouldn't detail what unions might be providing support, saying the campaign was still working that out.

"We ask people to volunteer for us for all kind of things," he said.

According to the e-mail sent to union leaders, the campaign was considering using paid security at some offices. Smith said that was something the campaign was talking about, but no decisions had been made. *

The Associated Press contributed to this report.