Miami (CNN) Sen. Marco Rubio cast himself as the forward-looking candidate to lead the nation toward a new American century when he announced his presidential bid Monday, framing the election as a "generational choice" for Americans.

"Grounded by the lessons of our history, but inspired by the promise of our future, I announce my candidacy for President of the United States of America," Rubio told supporters at Miami's Freedom Tower.

The 43-year-old freshman senator is the youngest contender in a rapidly-growing race for the presidency, and his speech Monday signaled he'll aim to turn his youth and relative inexperience into a central calling card of his campaign. That offers him a clear and immediate point of contrast with two of the top contenders in the race, both former Secretary of State Hillary Clinton, who became the first Democratic candidate with her launch on Sunday, and likely GOP contender Jeb Bush.

Rubio wasted no time in taking a swipe at Clinton.

"Just yesterday, a leader from yesterday," he said, as the crowd erupted in boos, "began a campaign for President by promising to take us back to yesterday. But yesterday is over, and we are never going back."

Rubio's candidacy will also lean heavily on his personal story as the son of Cuban immigrants. In his speech, he told the story of his parents' struggles and said that today, the American Dream seems unattainable for many because "too many of our leaders and their ideas are stuck in the twentieth century."

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"My candidacy might seem improbable to some watching from abroad," Rubio said. "In many countries, the highest office in the land is reserved for the rich and powerful. But I live in an exceptional country, I live in an exceptional country, where even the son of a bartender and a maid can have the same dreams and the same future as those who come from power and privilege."

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But while Rubio has all the makings of a winning presidential candidate — youth, a compelling personal narrative and deep roots in a crucial swing state — he's still missing one thing: Support.

The senator consistently polls in the middle of the GOP primary pack, trailing fellow Floridian Jeb Bush, Wisconsin Gov. Scott Walker and even more conservative alternatives like Texas Sen. Ted Cruz.

Supporters dismissed concerns about his middling polling position and relatively thin resume after hearing him speak.

Alfredo Ortega, the chief of staff for the city of Doral, Florida, said he believed because Rubio is a "phenomenal politician" he could overcome many of the hurdles he currently faces.

"I totally believe now is the right time," Ortega told CNN at the event. "If he waits for another term it might be too late for him. So I'm positive he's going to turn the polls around and he's going to become president."

Ortega, a Venezuelan-American who came to America just 11 years ago, said he saw Rubio's ability to win support from both the "Anglo and Hispanic worlds" as one of his major assets. And he was so impressed with Rubio's speech, he said, he'll be calling to sign up to volunteer for the campaign soon.

And it's that eloquence, said Florida GOP strategist Rick Wilson, that will help Rubio overcome the "static" in the field right now, as he jockeys for an advantage over already-announced candidates like Cruz and Sen. Rand Paul of Kentucky.

"Marco Rubio has a certain fluency, and ability to connect with voters, and this will be one of the first times a wider audience has seen it. When people who haven't seen Marco speak finally see Marco speak, they stand back and say, 'Whoa — what have I been missing?'" said Wilson.

His first test as an official candidate alongside other announced and prospective candidates will come this weekend, when he joins nearly the entire GOP field at the the Republican Leadership Summit. Prior to that, he'll stop in New York City for fundraisers on Thursday, and will spent Tuesday and Wednesday back in Washington focused on Senate business.

Rubio now faces the challenge of maintaining establishment interest while growing his appeal among conservatives. The right remains wary of Rubio for partnering with Democrats on an ultimately failed immigration reform bill after riding into office on the 2010 tea party wave.

Just right

But in the middle is just where Rubio wants to be.

He's hoping to carve out a path in the crowded GOP field as the candidate who's not too far right to lack credibility nationally, and yet not too squishy on conservative priorities like guns and same-sex marriage to lose the GOP base.

Rubio has eschewed the bomb-throwing of conservatives like Cruz and Louisiana Gov. Bobby Jindal, and the head-spinning vacillations of some of the party's more establishment-minded candidates in favor of what supporters see as a candid pragmatism. His career in politics — which began with a stint as West Miami city commissioner and found him ascending to House speaker before his surprise U.S. Senate win in 2010 — has been centered on that pragmatism, supporters say.

He was the first in the 2016 field to declare, without reservations, that parents "absolutely" should vaccinate their children. On same-sex marriage , he said if the Supreme Court rules it's constitutionally protected, "we have to abide" by that ruling. He repeatedly asserted that it was impossible to defund President Obama's immigration executive order by shutting down the Department of Homeland Security.

That strategy means he may not win any of the early nominating states that typically winnow down the field, but the senator and his advisers are banking on top-three finishes that keep him in the game until the flaws in the rest of the field are apparent and voters are looking for another option.

But it doesn't come without risks. One of Rubio's biggest obstacles to the nomination remains a persistent skepticism among conservatives who believe he sold out by taking a lead role in negotiating the 2013 immigration reform bill, which died in the House after Rubio helped secure its passage in the Senate.

In doing so, he plummeted from Tea Party darling and GOP rising star to freshman has-been in a matter of months, facing hecklers at town hall meetings and declining approval ratings in his home state.

Still, he's made light of the controversy, joking at a February event that he's the rare elected official to be heckled "by both sides of the immigration debate." And the gambit won him enduring respect among many establishment Republicans, who admire his willingness to compromise and cut deals.

Foreign policy prowess

Where Rubio has seized the spotlight, however, he's made it count.

Rubio led the charge earlier this year against the Obama administration's move to thaw relations with Cuba, appearing on nearly every major news network to slam the policy shift as shortsighted and scheduling hearings as chairman of the Senate Foreign Relations Western Hemisphere Subcommittee on the issue.

The potential advantages for Rubio in the Cuba issue were varied. It allowed Rubio to introduce himself to an entirely new audience of Americans, as the historic policy shift drew front-page headlines nationwide. It gave him an opportunity to emphasize his compelling personal narrative as the son of Cuban immigrants who left the country due to many of the same hardships imposed by the Castro regime that Cubans continue to experience today.

And it was yet another chance for Rubio to stake a claim as the GOP's strongest candidate on foreign policy, which Republicans believe will become a defining issue in the primary and general elections.

While Rubio hopes to wind his way down the middle of the GOP, he and his advisers believe foreign policy will be his calling card in the 2016 election: The issue that piques voters' interest keeps them coming back.

The freshman senator has been working since his early days in office to build the experience to make that case. He's taken 12 trips abroad since 2010 and co-sponsored more than a dozen bills on issues ranging from Russia's aggression against Ukraine to humanitarian crises in Haiti. He is a frequent and prominent critic of Obama's plan to tackle ISIS, and has slammed the Iran deal as insufficient to prevent the nation from gaining nuclear weapons.

Rubio has weekly briefings on foreign policy issues with leading advisers in the party, and aides say he's a voracious reader of policy papers and international news.

Sunshine State showdown

But first, he'll have to maneuver his way through the rocky and complicated primary terrain, and his launch puts him on a certain collision course with another Florida politician: former Gov. Jeb Bush.

Bush holds a slight lead over the field in early polling and remains the preferred pick for most of the GOP's establishment wing, and has already locked up a sizable chunk of the party's big donors, despite being still weeks away from announcing.

With Florida moving to a winner-take-all primary system, the Sunshine State showdown on March 15 could prove to be a pivotal contest for both candidates, and Bush's team is reportedly already strategizing to lock up the state's primary vote.

But Rubio's already shown that Bush will be less of an obstacle than many initially expected, as he's managed to post strong fundraising totals despite competing with Bush for hometown dollars.

Before Rubio makes it to Florida, however, he'll have to stay in the game for the party's early nominating contests. Though he's not yet the clear pick for voters in Iowa, New Hampshire or South Carolina, he's hired a top-notch team of strategists to steer him through the primaries there.

Those include Todd Harris, a Rubio campaign strategist, who advised Jeb Bush in the early 2000s before helping drive Rubio to an upset Senate victory in 2010; Terry Sullivan, who's running Rubio's PAC and previously ran former South Carolina Sen. Jim DeMint's campaign; and Jim Merrill, Mitt Romney's New Hampshire guru, who's serving in the same role for Rubio.