you are assuming he will get all those votes that voted democratic last time. I wouldn't assume so.
Alrighty I'll get a little more technical. (I Intentionally oversimplified it.) There are a number of variables that are wild cards, including the one you mentioned. Most variables have strong reasons why they can change to help, hurt, or stay the same. In regards to democrat votes, there are many democrats who live in this district who don't bother voting because there is generally no point. They might come out when it's a close election.
Another variable, already mentioned, is the increased number of Lakewood residence making the effort to vote, boosting turnout. People could care less about voting for Thompson. There have been specific efforts in the growing chassidish community that can play a role as well. There may may also be a protest vote against Schnall by the "Howell strong" crowd.
There are plenty of counterarguments, as well as other arguments to be made. I don't have enough conviction in any to conclude that they in aggregate help or hurt Avi. Which is why I oversimplified it.
What are the reasons someone would not vote for him other than the fact he is running as a democrat? He hasn't yet been tarnished as being part of the elected establishment and he is running against someone who hasn't done anything for the Lakewood community.
There are a lot of people in Lakewood who are instinctually distrustful of anybody the Vaad endorses, and the Vaad also endorsed Avi.