Obama told supporters the video of Trump joking about groping women had "shaken me to my core."
The comments come nearly a week after the 2005 video surfaced and amid a round of new allegations from several women who said Trump assaulted them by touching and kissing them when they were alone.
Two women told The New York Times that Trump tried to force himself on them, one on an airplane and the other inside Trump Tower in New York in the mid-2000s. A third woman, an entertainment reporter for People magazine, said she was sent to interview Trump and his wife, Melania, who was pregnant at the time. She said that while Melania was out of the room Trump kissed her against her will and boasted the two were going to have an affair. A Florida woman also told the Palm Beach Post that Trump grabbed her buttocks inside his Mar-A-Lago club, when she was working as a photography assistant.
Trump categorically denied each of the allegations, threatening to sue the Times for libel. The paper's legal department released a letter responding to Trump's lawyer in which they denied his demand they retract the story and apologize. It said, in part "we welcome the opportunity to have a court set him straight" if Trump pursues a libel suit.
Speaking about the accusers, a Trump spokeswoman interviewed Wednesday night on CNN said the women are seeking their "15 minutes of fame."
At a rally in West Palm Beach, Fla., on Thursday, Trump said the women are part of a coordinated effort between Clinton and the media to spear his reputation ahead of the election.
"The most powerful weapon employed by the Clinton political machine is the corporate media, the press," Trump said. "For them, nothing at all is out of bounds."
Democrats have pounced on the allegations, with Clinton and her top surrogates declaring his candidacy a fraud.
In Sunday's debate, Trump answered questions about the video, saying it amounted to "locker room talk." When pressed by the moderator about whether his conduct had ever progressed beyond lewd words into action, Trump flatly said "no."
Obama, who has become one of Clinton's most powerful surrogates because she is widely liked and trusted by voters, cast the Republican nominee as a predator preying on unsuspecting women.
"This was not just a lewd conversation. This wasn't locker room banter. This was a powerful individual speaking freely and openly about sexually predatory behavior," Obama said.