Since the 1947 constitution, governors have appeared on the ballot 17 times. In 10 of those elections, the governor's party has either gained seats or at least maintained their level of party support in both chambers of the legislature. In four instances, the governor's party had mixed success, gaining seats in one chamber, while losing seats in the other. And only three times has a governor won election while his (or her) party lost seats in both chambers. On average, the governor's party tends to pick up one or two seats in the Senate and about six seats in the Assembly.
An election during the middle of a governor's term is a much different story however. Midterm legislative elections have been contested 15 times since the new constitution was put in place, and almost always punish the governor's party.
The last time the governors party gained seats in both chambers was in 1951, and the last time the governor's party did not lose at least a seat in one of the chambers was in 1955. Over the past 55 years, every governor has seen his or her party lose seats in at least one of the chambers, and typically both. On average, sitting governors see one or two of their party colleagues depart the Senate, and nearly eight depart the Assembly.
This pattern has changed over time though, making midterm elections somewhat safer for the governor's party in recent decades. Since the landslide rejection of Democrats in 1991, New Jersey voters have not removed more than one senator or three members of the Assembly in any midterm election.
Redistricting, presumably, has played a large role in returning incumbents safely back to the legislature, as has the decline in voter turnout. Although nearly half of the electorate turned out to vote for the state legislative elections 20 years ago, barely a third has since. Those who do vote are typically more heavily partisan, consistently voting for the members of a single party and keeping the partisan balance in the legislature more constant.
What will this year's midterm election bring? Given the incumbent-friendly redistricting plan adopted by the state over the summer, it is unlikely that the legislature will change to any great degree. While historically it would seem more likely that the Democrats could pick up seats in the legislature -- given the presence of a Republican governor -- this year may buck that trend. The adopted legislative map protects almost all existing officeholders from both parties, making it unlikely that either party will lose or gain many seats. This could lead to a new trend in New Jersey politics, with the legislature stably controlled by the Democratic party for the coming decade.