In New Jersey, Republican Chris Christie pulled off an upset to defeat the incumbent Jon Corzine, by a margin of 55-44, according to preliminary results.
The Republicans also won back the governorship of Virginia.
In New York, Mayor Michael Bloomberg has won a third term in office after engineering a change in the city's term-limit laws so he could run again.
However, local media say his margin of victory over Democrat Bill Thompson looks set to be much narrower than expected at about 50% to 47%.
Mr Bloomberg also used his personal fortune to spend a record amount on his campaign.
Described by Forbes magazine as the richest man in New York, with a $16 billion fortune, Mr Bloomberg has spent almost $90 million on his re-election bid and is on track to spend as much as $140m overall. Mr Thompson spent $7m.
