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Bill Foster has won 52 percent of the vote compared to 48 percent for Republican Jim Oberweis with 556 of 568 precincts reporting.
Foster's special election win on Saturday means he will fill the remainder of Hastert's term until it ends next January.
The 66-year-old Hastert, a Republican who lost his powerful post as speaker when Democrats took control of Congress, resigned late last year.
Another election for the seat will be held in November for a new, full term.
Oberweis is the Republican candidate. Foster won a close Democratic primary, although one challenger has initiated a recount.
Source: DDNEWS