Kevin Keenan, who directs the ACLU in San Diego and Imperial Counties says his office sent an employee to the Arizona office to help interview plaintiffs for the lawsuit.
Keenan says he wants to make sure Arizona's immigration law SB1070 stops in Arizona.
"We're afraid about a spillover effect to California and particularly the border region. We want more people to understand that this isn't just about the immigration debate, that this law is much more dangerous for a range of civil liberties issues," says Keenan.
The ACLU's suit to stop Arizona's law says SB1070 essentially requires police to rely on racial profiling to determine who is undocumented. Keenan says that makes anyone with brown or olive skin a target.
The suit also says Arizona's law violates the first amendment's guarantee of free speech.
The law could take effect in July.