It's no surprise that Star Trek: The Original Series struggled in the ratings for the entirety of its run. Though season one has some of the best stories and strong characterization and the ratings were relatively strong, they didn't have the consistency needed to guarantee a season two renewal. There were even rumors that NBC was flirting with the idea of cancellation.
Gene Roddenberry reached out to prolific sci-fi writer Harlan Ellison for help. Ellison had, at the time already sold scripts to many television series, had written a novel, edited Rogue Magazine, and published more than 100 short stories and articles. He was a force to be reckoned with in the sci-fi industry. So it made sense that Roddenberry would seek his help. According to The Fifty Year Mission The First Twenty-Five Years by Edward Gross and Mark A. Altman, Ellison wrote a letter on December 1, 1966 to the Science Fiction Writers of America, only four months before the episode he'd written for Star Trek, "The City on the Edge of Forever" debuted.
Ellison told the SFWA that "Star Trek's cancellation would be tragic, seeming to demonstrate that real science fiction cannot attract a mass audience. We need letters! Yours and ours, plus every science fiction fan and TV viewer we can reach through our publications and personal contacts." As we all know, Ellison's intervention worked, and the science fiction fandom got to work to save Star Trek for another season.
Unfortunately, any good will Roddenberry had with Ellison dissipated after his script for "The City on the Edge of Forever" was rewritten. Ellison had a reputation for being abrasive and argumentative, and he didn't take kindly to his work being rewritten. The relationship between him and Roddenberry turned acrimonious after that, and Ellison never wrote another script for Star Trek. But we do still have him to thank for helping Star Trek get to season two.