Cesar Sayoc, 57, who was arrested in Florida in October, pleaded guilty to 65 counts and could face life imprisonment at his sentencing in September.
Appearing in court in a blue prison jumpsuit, and with his hair pulled into a gray ponytail, he apologized for his actions during an appearance in a US District Court in New York.
Reading a statement prepared with his lawyers, he admitted he fashioned the homemade devices from materials including plastic piping, a digital alarm clock with electric wiring attached, fireworks and glass fragments.
"I know these actions were wrong and I am sorry," he told the court, in a barely audible, choked voice.
The slew of charges relates to 16 package bombs Sayoc sent from a Florida post office to leading Democrats as well as the Manhattan offices of CNN.
Besides Obama and Clinton, those targeted included billionaire George Soros, former president Bill Clinton, former vice president Joe Biden, actor Robert De Niro and several Democratic lawmakers, including 2020 presidential hopefuls Cory Booker and Kamala Harris.
None of the packages exploded and most did not come anywhere near their intended recipients.