The bids include 143 regional jets flown by bankrupt Delta subsidiary Commair and other Delta and non-Delta carriers flying under the Delta Connection name, the Cincinnati Enquirer reports.
The routes include Delta hub cities such as Atlanta, New York and Cincinnati.
The decision affects Commair, based in Erlanger, Ky., more than any other carrier.
It puts 27 70-seat Commair jets as well as an undetermined number of 50-seat planes flown by Commair into play.
If Commair wins none of the bids, its fleet could be shrunk to 117 or less, the newspaper reports.
Commair is scheduled to have concession talks with its flight attendants and pilots unions this week.
Union officials could not be immediately reached for comment on Delta's decision.