The New York Times reports Pirro's first day on the campaign trail Wednesday was marred by minor logistical problems and by questions about her husband, who served a prison term for tax evasion.
Pirro positioned herself as an independent-minded Republican who disagrees with the party's positions on abortion and other social issues.
"I'm Republican red on fiscal policy with conservative beliefs on making tax cuts permanent, but I've got broad blue stripes on the social issues that don't change based on the office I am running for," Pirro said.
While warning Clinton to run a positive campaign, Pirro constantly referred to her by her first name. She also accused Clinton of using the Senate seat as a springboard for a 2008 presidential run.
Three other candidates are also seeking the Republican nomination.