Cardin's office announced Thursday that the senator sent a letter to Holder on the day after the election calling for the investigation.
Cardin, a Democrat, wrote that misleading automated phone calls were clearly intended to suppress voter turnout in predominantly African-American communities.
Robo calls made Tuesday afternoon before the polls closed urged voters who support Democratic incumbent Gov. Martin O'Malley to stay home, because the governor already had won.
"I'm calling to let everyone know that Governor O'Malley and President Obama have been successful," says a woman's voice on the recording. "The only thing left is to watch it on TV tonight. Congratulations and thank you."
NBC4's Chris Gordon reported that it was from a 202 area code. The O'Malley campaign said the calls were reminiscent of Republican candidate Bob Ehrlich's negative campaigning in 2006, when he lost his re-election bid.
Ehrlich's campaign said it had nothing to do with the calls and condemns them and anything else that attempts to mislead voters, NBC4's Darcy Spencer reported.
O'Malley ended up defeating former Gov. Ehrlich.