Pellets from a BB gun left marks next to re-election signs and familiar slogans posted at the office, located along the 6700 block of South Centinela Avenue in Westchester.
Three front-door windows and a side glass door were smashed. Plywood boards covered up some of the damage Friday.
Investigators said they did not find rocks or projectiles at the scene. They believe one or more attackers used BB guns to cause the damage.
Detectives believe the attack was politically motivated because no other businesses in the usually low-crime area were targeted. No threats were made before or after the attack.
A Democratic official told Eyewitness News there were three staffers and one volunteer in the office at the time the glass broke. No one was injured. It was originally reported that no one was inside the office at the time of the incident.
Some employees of the building were near the parking lot when they heard glass breaking just after 7 p.m. Thursday.
"It could have been a kid's prank or anything," said Joyce Rankin, who works at a collection agency on the third floor of the building. Rankin was not in her office when the vandals fired the BB pellets and she's relieved nobody was hurt.
"They could seriously have hurt someone if it had been something other than a pellet gun or whatever they could have possibly used. To me, it makes no sense. It's just a piece of property," said Rankin.
Police do not have a description of the vandals or surveillance video of the crime.
The attack comes just days ahead of the president's scheduled visit to Southern California.
Obama is expected to arrive in Los Angeles Monday to meet supporters at the House of Blues in West Hollywood, then attend a private event nearby. The money raised is for his 2012 campaign.
The president last visited Los Angeles in April also to attend a pair of fundraisers.
The Secret Service has been notified of the incident, and FBI officials said they are assisting LAPD with the investigation.
If you have any information regarding this incident, you're urged to contact the LAPD at (877) LAPD-24-7 or (877) 527-3247.
The Associated Press contributed to this report.