Sam Pimm, a veteran conservative New Hampshire operative, previously served as executive director of the pro-Carson super PAC the 2016 Committee, but officially made the swap on Sunday, he told CNN.
were first to report the news.
Also Sunday, all the remaining New Hampshire PAC staff resigned from their posts and publicly endorsed the Texas Republican.
Though Pimm left his post at the Carson-supportive super PAC before then, he had remained loyal to Carson until this weekend.
"I admire Dr. Carson very much, I agree with his policy stances, he's a very intelligent, bright man, but it's not in the cards that he's going to be the nominee of our party in 2016," Pimm told CNN. "I think Sen. Cruz has the best opportunity to win the nomination and get elected president."
Pimm said he believes his switch and the departure of the other 2016 Committee staff show a rising tide for Cruz in New Hampshire, the second state to hold a vote or caucus in the primary.
"I think it's going to foreshadow a big move toward Ted Cruz on the ground here," Pimm said. "When you say the primary is three weeks away, that is an eternity, and I see movement here among grassroots conservatives toward Sen. Cruz. I think he'll run very well here."
A Monmouth University poll released Monday
showed Cruz polling surprisingly strong in New Hampshire, traditionally a state that picks moderate Republicans, coming in tied for second to Trump at 14%.
The news also comes as Carson has shaken up his campaign. After talk of changes by Carson over the Christmas holidays, which was then walked back, Carson's top campaign staff including his campaign manager and spokesman left the team. Carson has replaced them with staff he says are better able to "execute."
After a surge in autumn, Carson has fallen in the polls and struggled amid a renewed focus on foreign policy and national security in the primary in the wake of the Paris terrorist attacks.
Pimm said he was looking to pick a potential conservative winner, but he said the front-running Trump did not appeal to him.
"(Cruz) has always been a conservative, his instincts are right and that counts a lot for me," Pimm said. "Donald Trump says some good things that I agree with, but whether he believes them or not, I don't know."
Neither the Cruz nor Carson campaigns responded to request for comment.