"I want to be helpful. I don't want to be harmful, because I don't want Hillary Clinton to be president," Rubio told CNN's Jake Tapper in an interview.
"Look, my policy differences with Donald Trump — I spent 11 months talking about them. So I think they're well understood," Rubio said. "That said … I don't want Hillary Clinton to be president. If there's something I can do to help that from happening, and it's helpful to the cause, I'd most certainly be honored to be considered for that."
He also said his political aspirations aren't over, calling it a "safe assumption" that he'll run for office — the presidency or something else — again.
"I can tell you I enjoy public service. If there's an opportunity to serve again in a way that I feel passionate about, I'll most certainly think I would explore it," Rubio said. "But I don't know where I'm going to be in two years. I don't know what my life will look like then."
The Florida senator's decision to embrace his former rival is the latest sign that top Republicans are embracing Trump, who has now racked up the 1,237 delegates necessary to clinch the party's nomination at its convention in Cleveland in July.
It's a remarkable reversal for a former 2016 presidential candidate who earlier this year tweeted using the #NeverTrump hashtag, indicating he'd joined the ranks of Republicans who would refuse to vote for the presumptive GOP nominee no matter the circumstances.
Asked specifically if he'd speak on Trump's behalf if Trump asked, Rubio said: "Yes."
He also said he'll release the 167 delegates he won during the primary, with victories in Minnesota and Puerto Rico and the District of Columbia. Rubio won the third-most delegates, behind Trump and Texas Sen. Ted Cruz — besting Ohio Gov. John Kasich even though Kasich stayed in the race several weeks after Rubio departed on the night he lost at home in Florida.
Rubio said he's not interested in being Trump's vice presidential running mate, saying he "wouldn't be the right choice" for Trump.
"He won the nomination and he deserves to have a running mate that more fully embraces some of the things he stands for," Rubio said.
Rubio, who was Florida's state House speaker before being elected to a single term in the Senate in 2010, isn't running for re-election and will depart office in January 2017.