Rubio, the breakout star of the 2010 election and a tea party favorite, kept a low profile early on in the Senate. That's begun to change.
In a matter of days, Rubio made his opposition clear in a Wall Street Journal article to raising the federal debt ceiling and he has called on lawmakers to authorize force to capture Libyan leader Moammar Gadhafi.
Rubio, the son of Cuban exiles, drew attention last year when he took on former Gov. Charlie Crist for the Senate. Rubio won the GOP primary and staved off Crist's independent bid.
The 39-year-old former Florida House speaker had followed a path forged by President Barack Obama and Secretary of State Hillary Rodham Clinton, both former senators: keep your head down when you reach the Senate, learn the ropes, don't make waves.
But aides said Rubio felt compelled to speak out as the Senate moved closer to considering a plan to raise the federal debt ceiling, a move most conservatives oppose.
"When he first got to the Senate, there was a lot to absorb very quickly," said Todd Harris, a campaign strategist for Rubio. "At the same time, there was no way he was going to allow the fight over the debt limit to come and go without playing a key role."
The Treasury Department estimates the government will reach the $14.3 trillion debt ceiling sometime between April 15 and May 31. The White House has warned that it could lead to a default on the national debt and harm the economy.
In his Wall Street Journal article, Rubio said he would vote against an increase in the debt limit unless "it is the last one we ever authorize" and it includes significant spending cuts, a balanced-budget amendment and reforms to Social Security, Medicare and Medicaid.
He also turned his attention to Libya, asking Senate leaders to advance a resolution authorizing Obama's move to join in military action in Libya. "As long as Qaddafi remains in power, he will be in a position to terrorize his own people and potentially the rest of the world," Rubio said.
When a spokesman for Senate Majority Leader Harry Reid, D-Nev., criticized the proposal, Rubio sent a letter to Reid asking him to reconsider.
"I understand that reflexively attacking the ideas proposed by another member of the opposing party has sadly become the way of the modern Senate," Rubio wrote.
Rubio's Senate office receives about 200 speaking and meeting requests a week, but the senator has limited his public speaking engagements to Florida. He turned down invitations to speak at Republican dinners in Arizona, California and Colorado, a tea party gathering in Idaho and an event at Harvard University, said his Senate spokesman, Alex Burgos.
Rubio discussed the debt ceiling and federal spending Wednesday at a weekly meeting of Americans for Tax Reform, a conservative anti-tax group headed by Grover Norquist.
"He brings significant stature. The other thing is that he's one of four obvious vice presidential candidates, and he's that without having to campaign," Norquist said.
Based on reporting by the Associated Press.