A staffer had to tell Bill Clinton about the infamous Trump-Rubio fight in 2016 over penis size

• Former President Bill Clinton missed the debate when then-candidate Trump bragged about his manhood.
• A staffer for Hillary Clinton's 2016 presidential campaign had to tell him about it.
• It was "one of the more awkward moments in my life," Josh Schwerin told Insider.

The moment when then-candidate Donald Trump boasted about his manhood during a 2016 Republican primary debate was particularly memorable for a staffer who had to brief former President Bill Clinton on what happened.

The former president had been in a meeting in Louisiana and missed the debate when Trump, responding to an attack from his opponent Florida Republican Sen. Marco Rubio, assured a crowd that neither his hands nor "something else" were small.

Josh Schwerin, who served as the national spokesperson for Hillary Clinton's 2016 presidential campaign, said briefing the former president on the debate was "one of the more awkward moments in my life."

"He didn't at first believe me that this was the topic of a debate," said Schwerin, who spoke to Insider as part of an oral history project on Trump's takeover of the GOP. "I had to show him the CNN headline. I tried to not add any commentary and just let him read it for himself. Because it was not the most comfortable conversation to have with the former president of the United States."

Read more: The definitive oral history of how Trump took over the GOP, as told to us by Cruz, Rubio, and 20 more insiders

Tired of Trump calling him "little Marco," Rubio seized on the size of Trump's hands that year during a rally in Roanoke, Va. "You know what they say about men with small hands? You can't trust them," he said.

Trump brought up the comment days later at the debate. "Look at those hands, are they small hands?" he said, holding up his hands for the crowd. "And, he referred to my hands - 'if they're small, something else must be small.' I guarantee you there's no problem. I guarantee."

Rubio later said he apologized to Trump for his comments.

Bill Clinton's extramarital sex scandals generated embarrassing headlines during his presidency and led to his impeachment. But the Rubio-Trump schoolyard antics still surprised him.

"He was amused, but also really aghast that this is what they had devolved to," Schwerin said of Bill Clinton.

To read the full oral history story, click here.