Beto O'Rourke said the best piece of advice he heard during the 2016 presidential election came from Sen. Bernie Sanders.

O'Rourke told Vanity Fair he recalls hearing Sanders tell Democrats that they had to have a bigger message than reminding "America how bad Donald Trump is."

"That moment sticks out to me so much, because it was so dramatic," O'Rourke said. "He said the most important thing that I'd heard during that entire campaign."

Beto O'Rourke, the latest member of an expanding field of 2020 Democrats, said the best advice he heard from anyone amid the contentious 2016 presidential election came from Sen. Bernie Sanders about then-candidate Donald Trump.

In the days leading up to the 2016 Democratic National Convention in Philadelphia, there was a lot of animosity toward Sanders from supporters of Hillary Clinton who felt he was dividing the party and working to sabotage her nomination.

O'Rourke, in an interview with Vanity Fair, recalled a meeting Sanders and Clinton had with congressional Democrats around that time in which the Vermont senator offered advice that could shape the Texas Democrat's approach to his 2020 campaign.

Read more: Bernie Sanders hits the 2020 campaign trail with rockstar status, a far cry from the start of the 2016 campaign

"He said it's not enough to remind America how bad Donald Trump is, it's just not going to do it," O'Rourke said. "You've got to give people something to be for, it cannot be who we are against."

"I think he was so prescient," he added. "That moment sticks out to me so much, because it was so dramatic. He was so hated really—it's not too strong of a word—when he was in there, and he said the most important thing that I'd heard during that entire campaign."

Read more: Bernie Sanders' 2020 campaign slogan is a direct rebuke of Trump's 2016 message of 'I alone can fix' America

In many ways, Sanders appears to have stuck to this perspective as he also vies for the 2020 Democratic presidential nomination.

During a recent speech in Council Bluffs, Iowa, Sanders pulled no punches when it came to Trump, referring to him as "the most dangerous president in modern American history."

But Sanders was also careful to avoid making this election, and his campaign, just about defeating the current occupant of the White House.

Read more: Bernie Sanders says fellow 2020 Democratic candidates are now running on his 2016 ideas that were originally dismissed as 'too radical'

"Make no mistake about it, this struggle is not just about defeating Donald Trump," Sanders said. "This struggle is about taking on the incredibly powerful institutions that control the economic and political life of this country."

As O'Rourke's campaign picks up steam, we could see him embrace a similar approach.