"Who is the Democrat running against him?"
It doesn't matter, since Kolbe's real opponent is conservative state representative Randy Graf. But since you asked, the following are the three candidates for the Democrat nomination, as per Politics1.com:
Eva Bacal (D) - Tucson School Board Member & Assistant Attorney General
Jeff Chimene (D) - Computer Consultant & Community Activist
Tim Sultan (D) - Ex-Congressional Aide & Ex-Technology Firm Executive