did I somehow miss it or are supposed to assume that Jack McCoy was re-elected as DA for New York?
Did you see last season's finale?
In a multi episode arc last season, Jack's opponent in the race for DA was sponsored by the (fictional) NY governor. The governor's wife was put in prison, and the gov himself forced to resign, as a result of events in the season's last episode.
So any DA candidate whose patron was that governor, would be doomed to lose the election. And so we're left to assume that Jack's opponent withdrew, and Jack was then elected (presumably unopposed).