The Post spoke with nine women who said they were aware of, or had directly experienced, unwanted contact by Halperin over a period dating to the mid-1990s.
They described conduct that ranged from grabbing women's hands to late-night phone calls and aggressive sexual propositioning. Several recounted episodes in which he rubbed his erect penis against them, which Halperin specifically denied in an interview.
Another woman who worked with Halperin said that "he did a lot of inappropriate and creepy things. He would suggest that you come to his hotel room at late hours. He would call you at 2 in the morning - and not to discuss anything work-related."
The women who spoke to The Post said Halperin was at his most aggressive during the 2004 campaign, when he was in charge of ABC News's political coverage. As political director for the Disney-owned network, he was empowered to select the embeds, a low-level position but an important step in a highly competitive business.
Halperin, in a brief interview Thursday, denied some of the specific accusations against him.
After leaving ABC News, he worked for Bloomberg Politics and NBC, and he co-wrote "Game Change," the best-selling chronicle of the 2008 presidential campaign, which was made into an HBO movie. He also co-hosted a discussion program on Bloomberg TV titled "With All Due Respect" and starred with "Game Change" co-author John Heilemann in a Showtime series about the 2016 campaign, "The Circus." Until Thursday, he was a regular panelist on MSNBC's "Morning Joe."
The fallout has been swift for Halperin, one of the most prominent political journalists in the country. Halperin was put on leave as a MSNBC contributor. A book in the works about the 2016 election has been dropped by its publisher, Penguin, while HBO canceled a miniseries it was working on with the journalist.
Halperin is one of a growing number of men in powerful positions in various industries - media, film and restaurants, to name a few - to find themselves in the spotlight after women have accused them of harassment in recent weeks.
In his apology, Halperin said that he realized he had a problem with the way he treated women, eventually seeing a therapist. He said his conduct improved at other workplaces.
"Those who have worked with me in the past decade know that my conduct in subsequent jobs at TIME, Bloomberg, NBC News, and Showtime has not been what it was at ABC," he said.