Donnelly has put the seat in play for Democrats since Mourdock upended veteran Sen. Richard Lugar in May's GOP primary. Mourdock has had trouble locking in support from moderate Republicans. Libertarian Andrew Horning will also take part in Monday's debate.
Indiana's Senate battle is one of about a half-dozen tight races across the county that will decide whether Democrats or Republicans control the Senate.
The second and final Senate debate will be Oct. 23 in New Albany.