Speaking with CNN's Jake Tapper, Rubio shared the reservations he has about Trump as the GOP standard bearer.
"Here's the situation we're in," Rubio said. "On the one hand I don't want Hillary Clinton to be the president of the United States. I don't want her to win this election. On the other hand, as I said, I have well defined differences with the presumptive nominee of the Republican Party. I intend to live up to the pledge that we made. But that said, these concerns that I have about policy, they remain, and they're there."
In recent weeks, Rubio has said he thought Trump's performance had "improved significantly." But in the interview with Tapper, Rubio said the unusual tone of the 2016 election made it difficult to support Trump, adding "I don't know if there is well established road map forward" in terms of backing the real estate mogul.
The Florida senator suggested that Trump should deploy surrogates that truly believe in his message, rather than change his political views to get GOPers who, like Rubio, are unenthusiastic about his candidacy to support him.
"He's the presumptive nominee but he would be best served by having surrogates who agree with him on these issues," he said. "My reservations about his campaign and my political differences with him are well documented and they remain and I think he would be best served by having people close to him and his campaign that are enthusiastic about the things he ands for."
Rubio also rejected the idea of a Trump-Rubio ticket, noting that he hadn't been approached by the Trump campaign to be the businessman's running mate.
"No, he needs to appoint someone that more fully embraces the things he stands for," Rubio said.
When Tapper pointed out that wouldn't be too late for Rubio to file for re-election to his Senate seat, Rubio shot down that proposition, too.
"It is for me," he said.