The AFL-CIO and several union-friendly groups filed a formal complaint against Wal-Mart on Thursday, charging that the retail giant is pressuring employees to vote against Sen. Barack Obama.

Citing a Wall Street Journal report, the complaint accuses Wal-Mart of pushing its workers to oppose Obama (D-Ill.), who supports a measure that would allow workers to form unions without a secret-ballot election. The complaint calls on the Federal Election Commission (FEC) to investigate whether Wal-Mart has broken federal election laws.

Among the groups filing the complaint are WakeUpWalMart.com, the advocacy group American Rights at Work, and the Change to Win coalition, which is led by the Service Employees International Union and the Teamsters.

AFL-CIO President John Sweeney said that Wal-Mart has “bullied” employees for years.

“Wal-Mart has shown exactly why our nation needs the Employee Free Choice Act,” he said.  “We must outlaw the kind of behavior for which Wal-Mart is famous and give workers a free and fair choice on whether to form a union.”

A Wal-Mart spokesman said that though the company opposes the bill, it has not told associates how they should vote.

“We believe if the FEC looks into this, they will find what we’ve known all along, which is that we’ve done nothing wrong,” said spokesman David Tovar. “The real issue here is protecting the rights of individuals to cast a private ballot when voting.”