Bev Harrison represents Hampton-Kings in the legislature but will seek the NDP nomination in the new riding of Hampton, which was created with the reduction of the number of ridings to 49 from 55.
Harrison said he's making the switch because he's impressed with leader Dominic Cardy and changes in the party.
"I see a new NDP that has matured," he said Friday. "It's a party that has modernized, become more realistic and is offering real solutions."
The 72-year-old politician was first elected as a Progressive Conservative member of the legislature in 1978 and re-elected in 1982.
He was defeated in 1987 when the Liberals won every seat in the house under former premier Frank McKenna, but was re-elected in 1999 and in every election since.
The former teacher and high school principal also served two terms as speaker of the legislature.
Harrison said that despite his experience, he wasn't being listened to in the Tory caucus. He said government isn't working if it is not listening to its members.
"Otherwise, we're really doing a top-down management thing and might as well simply elect executive government of some kind," he said.
Jesse Robichaud, a communications officer for Premier David Alward, said the premier would not be available to comment on Harrison's switch to the New Democrats.
Jason Stephen, president of the New Brunswick Progressive Conservative Party, said he wasn't surprised by Harrison's move.
He said Harrison felt the party wasn't supporting him when it refused to block anyone else from challenging him for the Tory nomination in the riding.
"But in fact we were saying that every riding is open and we feel very comfortable with the transparent, open process with all of our riding nominations," Stephen said.
There is also some controversy over the New Democrats' nomination process for the Hampton riding.
Chris Rendell was set to seek the nomination, but has since been disallowed by the party.
Cardy said he called Rendell to ask if he would step aside and support Harrison.
"I asked Mr. Rendell if he was willing to give backing to a candidate who clearly brought a lot to the table and he said no," Cardy said.
"He was entirely free to go ahead with the nomination up until the point that he decided he was not going to support the party publicly, which is unacceptable in any party."
Stephen Yardy, the NDP's executive director, said Rendell had "liked" social media comments that endorsed another political party while attacking the New Democrats.
"I confronted the individual and when asked about the comments he was liking he said he was going to continue doing so, so therefore I could not approve his nomination going forward," Yardy said.
Rendell could not be reached for comment, but on his Facebook page said he had shared a Green party Facebook post and apparently hit like.
"The reality, as I am sure you know, is that they wanted to clear the way for another candidate to run in this riding," Rendell wrote.
The NDP has not set a date for the Hampton nomination meeting.