MIAMI — As Sen. Marco Rubio stepped briskly down the sun-splashed steps of the iconic tower here where he would later deliver his first speech as a candidate for president, he was greeted by a pack of reporters curious about how he would try to distinguish himself from others on the long list of 2016 hopefuls.

“Senator, why are you uniquely qualified?” one asked after reports that Rubio privately told donors that he was just that, “uniquely qualified” to be the GOP nominee.

That question, which Rubio sought to address in his kickoff speech Monday evening by drawing on his biography and his outlook on the future, will become increasingly important to his political future as he barnstorms the country in the coming months as an underdog with whom many Republican primary voters are unfamiliar. Complicating his task is a quickly growing field of opponents who have already sought to carve out unique niches.

Rubio, the son of Cuban immigrants, said in his remarks that his parents achieved the "American Dream," but that "the problem is now, too many Americans are starting to doubt whether achieving that dream is still possible."

The Florida Republican is the latest entrant into a primary field that looks as wide open as any in modern history. With polling showing that many voters have yet to form an opinion about Rubio, his challenge will be to define himself as a unique alternative to opponents who have already tried to set themselves apart from the pack.

Rubio’s quest started here Monday evening at the stately Freedom Tower, a building architecturally inspired by a Spanish cathedral and which is known as the “Ellis Island of the South” because it is where Cuban refugees fleeing the Fidel Castro regime were once processed.

In his kickoff address, the senator sought to highlight his humble roots in an effort to stand out.

"I know my candidacy might seem improbable to some watching from abroad. After all, in many countries, the highest office in the land is reserved for the rich and powerful," he said. "But 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."

Two hours before Rubio’s speech, supporters were lining up along the palm tree-lined front of the building as U2’s “Where the Streets Have No Name” played in an overflow area across the street where a smaller crowd watched his speech on a big screen.

Supporters said Rubio is a superior messenger compared to other candidates in the field.

"I think Rubio is sharper. And he gets it. And he's quick. Quick, quick, quick," said Karen Bailey of Ocala, Fla. Bailey said she met Rubio unexpectedly during a 2014 vacation to Hawaii.

As he officially kicks off his campaign, Rubio, 43, enters a fray already populated by Republican candidates with distinctive qualities.

There is former Florida governor Jeb Bush, who has quickly become the establishment favorite as he has moved quickly for big donor support and is running as an unapologetic center-right candidate unwilling to bow to conservative pressure on education and immigration.

On Bush’s heels in the eyes of many is Scott Walker, the Wisconsin governor who made his name as a conservative crusader by curtailing the influence of public sector unions in his state.

And those two aren't even official candidates yet. Two others who are — Sens. Ted Cruz (Tex.) and Rand Paul (R-Ky.) each have loyal tea party followings.

In this scrum lies perhaps Rubio’s greatest strength and his biggest weakness. He can lay claim to all of these camps but is not seen as the standard-bearer for any one.

Like Paul and Walker, he was elected to his current office as part the tea party wave election of 2010. Like Bush, he has flashed moderate leanings on immigration.

According to a recent Washington Post-ABC News poll, 38 percent of the public has no opinion of Rubio, a higher percentage than those saying the same thing about Bush, Cruz or Paul.

Democrats sought Monday to define Rubio as indistinguishable from the rest of the Republican candidates. In a hotel ballroom not far from the site where Rubio spoke, Democratic National Committee Chairwoman Debbie Wasserman Schultz (Fla.) accused him of lacking any fresh ideas on gay rights, climate change or abortion.

“When you have a prune that’s wrapped in tinsel it doesn’t make it fresh and new," she said.

In a statement, Cruz praised Rubio but also sought to remind voters that some of his qualities are not unique.

“We’re both the sons of immigrants who escaped Cuba to build a better life in the United States, and we share a deep appreciation and understanding of what it means to work hard and achieve the American Dream,” said Cruz. “Marco is a talented communicator and part of a new generation of Republicans stepping forward to promote conservative solutions to our pressing challenges.”

As he prepared for his speech Monday, Rubio addressed donors at a breakfast meeting held in a nearby hotel and he posed for photos before placing a call to a wider network of donors unable to be in Miami, according to a person who attended the breakfast.

On Sunday evening, Rubio held an intimate “friends and family” barbecue at the Miami home of his friend, Bernie Navarro, a real estate investment firm executive. Rubio’s wife and children, some of his siblings, many of his top staffers and other Miami-area friends attended, according to people familiar with the gathering.

Exiting Rubio's speech, Kelly Steele of Palm Beach, who was sporting a "Viva Marco" button, said part of Rubio's appeal is rooted in his not being tied to a dynastic political family.

"Do we need more of the Bush dynasty? No. I think we need some fresh ideas, some new blood," said Steele.

Ed O'Keefe contributed.