Trump previously promised not to bring a suit against Cruz.
O'Malley's March performs on March 13 following his unsuccessful Oval Office bid.
Ted Cruz is in second, 17 points behind, followed by Marco Rubio and Jeb Bush.
Priorities USA Action is spending $5 million on behalf of Clinton.
"It's the usual difference between political rhetoric and political deed," he said.
The suit was filed in federal court in Alabama on Feb. 3.
"I have no memory of it," the actress added.
"Look past the boasting, and you'll see right through him," the ad says of Trump.
Reince Priebus added that the Democratic presidential primary is "a train wreck."
The group is spending $1.5M on an ad that says there's "nothing conservative" about Trump.