Struggling through its worst-ever budget crisis, Chicago has suffered the first in what could be a series of body blows: a downgrade in the all-important bond-rating that determines city borrowing costs. Citing a record, $654.7 million budget shortfall and Mayor Daley’s “accelerated use of reserves to balance operations,” Fitch Ratings has reduced its rating on $6.8 billion in outstanding general obligation bonds from AA-plus to AA.