Altschuler leads Bishop, a Democrat, by about 400 votes, both campaigns said, citing data from the Suffolk County Board of Elections. That represents a swing of almost 4,000 votes because Bishop was ahead by 3,461 earlier this week. A senior elections official, who requested anonymity, confirmed the numbers.
Still to be counted are absentee ballots. Of 13,000 issued, 9,901 have been returned, including 3,611 from registered Democrats, 3,953 from Republicans and the rest from either minor party members or the unaffiliated.
Democratic elections Commissioner Anita Katz declined to comment due to pending litigation involving the House race. GOP commissioner Wayne Rogers declined to comment because he was dining with his family, adding he would discuss the matter Monday in working hours.
Bishop aide Jon Schneider said, "there was a wild miscount of the ballots [on election night] . . . we intend to get to the bottom of it."
Altschuler spokesman Rob Ryan said, "Today's turn of events reaffirms what we said on election night - this race is far from over."
The House race is one of several that remain too close to call. All appear to have been affected by this year's new voting system, where paper ballots and optical scanners replaced the old lever-voting machines.
Elections experts speculated the large swing in the Bishop-Altschuler vote tally may be because inspectors misread printouts from the scanners and relayed the incorrect information to the board on Tuesday night.
The reaction from party leaders was mixed. Edward Walsh, Suffolk Conservative chairman and an early Altschuler backer, said, "We're back in the game."
Suffolk Democratic chairman Richard Schaffer said, "We're back in a situation where we're going to have to count every single vote" by hand.
With Rick Brand, Reid Epstein and Dan Janison