Club for Growth is spending $100,000 on an ad that will run in Iowa and South Carolina.
Her unfavorable ratings are up 6 percentage points, but she still tops Republicans.
Walker polls high among hopefuls, though he has yet to officially declare.
"People like to hear me speak,” Clinton said.
Fiorina launched her bid with attacks on Hillary Clinton.
John Podesta said the campaign will not let attacks "go unchallenged."
Romney criticized remarks tying bias in the judicial system to economic inequality.
“It was something that I did completely unrelated to my job," the SEIU employee said.
“I think that she is the most likely person to be president," the billionaire investor said.