During a conference call with donors, Rubio criticized Democratic front-runner Hillary Clinton as a leader from yesterday and said the 2016 race will be a choice between the past and the future, the source said.
Rubio, a son of Cuban immigrants who rode the anti-establishment Tea Party wave of 2010 to national prominence, formally announced his presidential bid later on Monday with a speech at Miami's Freedom Tower.
That is where thousands of Cuban exiles fleeing the communist-run island in the 1960s were first registered by U.S. authorities. Rubio is expected to make a muscular foreign policy a focal point of his campaign, portraying himself as the Republican most ready to handle threats to America in a chaotic world.
Rubio's support registers in single digits in opinion polls of the likely contenders in what is expected to be a crowded Republican presidential field. But aides believe Rubio, who was on 2012 nominee Mitt Romney's short list for vice president, will rise when voters take a closer look at him.
He will be the third Republican to formally announce a White House bid, following Republican Senators Ted Cruz of Texas and Rand Paul of Kentucky.
During the campaign, Rubio probably will compete for donors and endorsements with his political mentor, former Florida Governor Jeb Bush, who has been lining up support for a White House bid although he has not yet formally entered the race.
Rubio will be competing for the limelight with Clinton, who grabbed worldwide media attention with the declaration of her candidacy for the Democratic presidential nomination in a video announcement on Sunday.
The former secretary of state will hit the campaign trail in Iowa on Tuesday and Wednesday. Iowa holds the kickoff contest in the parties' presidential nominating process early next year.
While he owes his success to the Tea Party movement, Rubio also has drawn support from more traditional party elements as well as the libertarian-leaning network assembled by billionaire Charles and David Koch.
His effort to overhaul the United States' immigration system could be a sticking point for Republican conservatives, many of whom view any move to grant legal status to undocumented workers as "amnesty."
Rubio worked with Senate Democrats to pass a sweeping immigration reform bill in 2013 that bolstered border security and guest-worker programs with a pathway to citizenship for those now in the country illegally. The measure died in the Republican-controlled House of Representatives.
Rubio now says any immigration reforms must be passed piece by piece, with border security coming first, a position more in line with other Republican lawmakers. But he talks frequently about the central role immigrants play in revitalizing the United States.