Except for Carlos Beruff.
The real estate developer with close ties to Florida Gov. Rick Scott has insisted on staying in the race until the end. Since announcing his candidacy in February, Beruff has pumped millions into his campaign -- efforts the wealthy businessman is paying for largely by himself -- and visited Florida's 67 counties. Along the way, he has aimed to establish himself as a political outsider and an enthusiastic supporter of Republican presidential nominee Donald Trump.
"I just think people are fed up with people who are career politicians who don't stand for anything," Beruff said in an interview. "I don't think Mr. Rubio represents the Floridians anymore."
Despite those efforts, Beruff's campaign has struggled to find its footing against Rubio, a popular candidate who failed to capture the GOP nomination this year but is well-known throughout the state. With only few weeks left before Florida's Aug. 30 primary, Beruff still trails Rubio by as many as 30 points, according to one recent poll.
Trump trounced Rubio in the Florida primary and Beruff has attempted to ride the nominee's coattails, capitalizing on voter frustration with the incumbent class. He aims to connect with Trump voters by hammering Republicans who are reluctant to embrace the nominee.
While many Republicans -- including Rubio -- tepidly endorse Trump, Beruff is seeking to scoop up hardcore Trump supporters by positioning himself as an enthusiastic champion of Trump's ideals.
"I will be a lieutenant to Mr. Trump," Beruff told voters in Fort Myers recently.
Beruff has adopted many of Trump's hardline policy stances. He wants to "close the border" with Mexico and he calls for a travel ban on visitors or immigrants from the Middle East. In one of his campaign ads, Beruff slams Rubio for not supporting Trump more enthusiastically, which ends with Beruff telling Floridians, "I am not ashamed to support Donald Trump."
Unfortunately for Beruff, Trump doesn't appear to have noticed his existence.
On a recent campaign speech in Daytona Beach, Trump made clear that he had moved on from his primary-era fights with Rubio and urged Floridians to support him.
"Go for Marco!" Trump told the crowd.
And while other conservative upstarts have received a financial lift from outside groups that challenge incumbents they deem insufficiently conservative -- like the Club for Growth or the Senate Conservatives Fund -- they haven't come in to back Beruff.
Rubio has largely refused to engage with Beruff, declining debate challenges and focusing energy toward the general election against a Democratic challenger. In July, America Rising, a Republican outside group allied with Rubio, began dispatching a tracker to follow Beruff to every public appearance with a video camera, a sign that Rubio isn't entirely writing off his challenger.
Meanwhile, a pro-Rubio super PAC called the Florida First Project produced an ad attacking Beruff for supporting former Florida Republican Gov. Charlie Crist, who has since switched parties and become a Democrat.
A Rubio campaign spokeswoman said in a statement that the senator is the candidate best positioned to overcome the Democratic candidate in the fall.
"The future of the Supreme Court, the Iran deal and the economy hinge on a conservative Senate," Rubio campaign spokeswoman Olivia Perez-Cubas said. "The direction of this country over the next six years could all come down to Florida's Senate race, and Marco is the only Republican who can win."
So far, this has not deterred Beruff, who continues to hammer Rubio regularly with a series of ad campaigns and media outreach.
Beruff's rise in real estate and Florida politics
Like Rubio, Beruff's parents are from Cuba and left the island before Fidel Castro's 1959 revolution. Beruff was born in the United States but briefly lived in Cuba as a child when his mother made a short return once Castro came to power.
He grew up within the Cuban exile community in Florida and went into real estate development. He lost his fortune in the early 1990s, but has spent the past 30 years earning millions in business, which he is now using to fund his campaign.
Although Beruff has never held elected office himself, he has contributed heavily to political candidates for decades. Beruff has developed close ties with Florida politicians, especially Scott, who appointed Beruff to the Southwest Florida Water Management Board. He has also held positions on the Sarasota-Bradenton Airport Authority and the State College of Florida Board. Beruff's history of political giving and government appointments have led Rubio allies to question whether he's really the outsider he claims to be on the campaign trail.
"If Mr. Trump is an outsider -- because he's given more money than I have -- then I suspect I'm an outsider too," Beruff said.
Some members of his staff include former aides to Scott, who has praised Beruff but not expressed support for Rubio's re-election bid.
"Carlos Beruff is a good friend of mine, a businessman and an outsider to politics," Scott said in June. "The voters of Florida deserve the opportunity to consider his candidacy alongside Senator Rubio and make their own decision."
With only a few weeks left before primary day, Beruff's campaign has not appeared to strike a cord with Florida's Republican voters, who overwhelmingly tell pollsters they plan to support Rubio. His audiences are usually small and while the ads have increased his name-ID throughout the state, it may not be enough to keep Rubio from becoming the nominee.
"I've been taking risks my whole life and this is no different," Beruff said of his campaign. "I don't need this career. I'm 58 years old. But this country has made me successful beyond anything I ever dreamed of. And at the end of the day if you don't make an effort to bring that back to the basics of this country, then the future for our young people isn't as bright as it was for me."