Rubio has been criss-crossing the country, attending small-bore fundraisers for potential allies in key states and holding policy events that barely register on the national radar. These type of events -- aimed at courting allies in early states and locking up big donors -- are all part of a strategic effort by Rubio to stay below the radar and avoid the scrutiny that comes with being a front-runner in the race.
Earlier this year, Rubio raised money for fellow Republican Senator Chuck Grassley, the dean of Iowa politics. And he also headlined a big money event for the senior senator's grandson, Patrick, a state senator.
The moves didn't generate much attention, and it didn't seem to matter much that Rubio has spent little time in Iowa in his five months on the campaign trail. Grassley was happy to see his warchest -- and his grandson's -- filled. "It would be hard for Grassley to say he's not spending enough time in Iowa," Grassley said with a big smile Tuesday.
On a day in early September when the political world was abuzz over a fresh clash between Jeb Bush and Trump, Rubio was raising cash in Oklahoma -- a state of less prominence in the presidential calendar than the first four early contests -- and talking about energy policy with business executives.
Tuesday, after campaign events in the South, Rubio slipped into the Senate to cast a vote reaffirming his opposition to abortion before catching a flight to Dallas for a high-dollar fundraiser at the home of real-estate investor Harlan Crow. His rival, Ted Cruz, stayed in the chamber and held court with reporters to lash Republicans for caving to Democrats.
In the summer of Trump, where candidates are making bombastic statements and dominating the headlines, Rubio has mostly tried to avoid drawing attention to himself, hoping others implode -- like Wisconsin Gov. Scott Walker did this week.
In the first two debates, Rubio didn't deliver a knockout blow, but his steady performance, polished answers about national security and his Cuban roots and lack of verbal gaffes helped his poll standing. While his 11% rating still is far off Trump, who remains in the lead, senior Rubio officials argue they are more than content seeing their candidate sitting quietly in fourth place at this stage of the race.
His rivals have seemed to noticed.
"I think he's running a good race -- he's doing better than I am," admitted Sen. Lindsey Graham, R-South Carolina, who is also running for president.
The strategy boils down to this: Pick and choose your spots, and -- for now -- stay away from the fray. And if Rubio continues to keep his head down, and avoids spawning much controversy, his campaign hopes a late surge could propel him to win the Iowa caucuses next year and win one of the first four early primary states.
Rubio, who declined to be interviewed, is publicly at ease with his standing in the polls. Flashing back to his 2010 Senate campaign in Florida, he said at an event in Charlotte this week: "Five years ago around this time I was 30 points down in the polls. The only people who thought I could win all lived in my home. Four of them were under the age of 10."
But like others, Graham was quick to warn such a strategy has its limits. "Leadership is engaging when you need to. You don't need to fight every fight but some of this stuff we need to stand up to."
Will face questions about his experience, immigration reform
Rubio is not without his vulnerabilities. As Fiorina's business record is starting to get new scrutiny amid her rise in the polls, Rubio's voting record and relative lack of experience are bound to get more attention as well. Both subjects offer ammunition for critics.
"Inexperience because of (President Barack) Obama's inexperience," Sen. John McCain, R-Arizona, said when asked about Rubio's vulnerabilities. "Look that would be one of the challenges."
While Rubio has had few legislative accomplishments in his less-than-one-term as senator, immigration is the issue that he's most identified with in the Senate. It's an issue that cuts both ways and could be dangerous for the candidate in both the primary and if he makes it to a general election.
Rubio cut a major immigration deal in 2013 that included a path to citizenship for the nation's 11 million undocumented immigrants, he abandoned the measure after it passed the Senate and stalled in the House. He has referred to his involvement in the issue as a mistake, saying Congress should instead try to pass individual pieces to reform immigrations laws, starting with enforcement.
But his handling of the matter has been a source of criticism for both sides.
Trump, in a Tuesday tweet, referred to him as "Senator Marco 'amnesty' Rubio," while proponents of immigration reform say the Florida Republican was quick to retreat when the going got tough.
"I was disappointed in that," said McCain, who worked with Rubio on the matter.
"It seems to me when you work with a group of people and you agree, you keep your word," Sen. Dianne Feinstein, D-California, said of Rubio. "That to me is kind of a danger signal."
Graham, another co-author of immigration reform, refused to single out Rubio -- but made a clear contrast with the junior Florida senator.
"Rather than me talking about what he did, let me tell you what I did: I stuck with it," Graham said. "I'm not going to change stripes on this issue."
It remains to be seen just how long Rubio can stay above the fray and if his standing will drop once the attacks start to sharpen. Few of his opponents have taken direct shots at him. Indeed, Cruz, the Texas Republican, at this month's CNN debate lashed "Washington Republicans" for backing the "massive amnesty" plan that Rubio backed, though he didn't single out his colleague by name.
In response, Rubio talked about his own immigrant family and laid out bite-size measures that should become law first, including tracking those who come into the country illegally, mandatory electronic verification of workers' citizenship and better border security.
"We cannot deal with all three of these problems in one massive piece of legislation," Rubio said at the debate. "I learned that. We tried it that way."
Rubio's contrition on the issue seems to have won him plaudits on the right, stabilizing his poll numbers and allowing him to exert himself on other issues, namely a more hawkish view of foreign policy. And his polished speaking style and youthfulness have made allies think that he could help bridge the divide between the party's warring wings and provide a sharp contrast to the eventual Democratic nominee, presumably Hillary Clinton.
At the same time, a top Democrat downplayed Rubio's strengths.
"He's one of the 15 as far as I'm concerned," said Senate Minority Leader Harry Reid, D-Nevada, when asked about Rubio, referring to the crowded primary field.
"I tell people it's like shopping for a car," said Sen. John Thune, R-South Dakota., No. 4 in the GOP leadership, referring to the primary contest. "People are taking these candidates out for a test drive, seeing what they like."
Rubio's campaign hopes voters don't test out his candidacy until next January.