Casey won Tuesday's Democratic Party primary over Joseph Vodvarka, a retired spring manufacturing shop owner.
With 33 percent of precincts reporting, Casey had almost 176,000 votes, or 80 percent, to about 44,400 votes for Vodvarka, or 20 percent.
Republicans are in a five-way race for the nomination to challenge Casey in the Nov. 6 general election. The candidates are David Christian, Sam Rohrer, Marc Scaringi, Tom Smith and Steve Welch.
