Patrick Ruffini wrote on Medium, Who Do Polls Say Will Win the GOP Nomination? Check Back in November. He goes through the 2004, 2008 and 2012 races and concludes:
"In no case did the eventual Republican or Democratic nominee exhibit strong momentum in the spring and summer the year before the election. In most cases, they lost support, and were either written off completely or had serious doubts raised about their ability to win. Candidates who appeared strong early in the process (Dean, Clark, Giuliani, Fred Thompson) only began losing support in the late fall or winter.
Crucially, the nominees’ turnarounds came as little as 30 days before the first nominating contests, and even then, these gains could have appeared uncertain and fleeting. But their gains in December were predictive of victories in January and February. Their numbers before November and December? Not as much."