DAVID GREENE, HOST:
Well, after saying for months that he wouldn't, Marco Rubio surprised almost no one when he announced he would run for a second term in the U.S. Senate. Despite his late entry into the race last week, Rubio became the immediate favorite to win his party's nomination. The field has been almost cleared. Rubio has just one major Republican challenger left. But NPR's Greg Allen reports that the challenger's a millionaire developer in the mold of another Republican who's popular in Florida - Donald Trump.
GREG ALLEN, BYLINE: If Marco Rubio has a political nemesis right now, it would be Trump. The developer trounced Rubio in his home state in Florida's primary, forcing him out of the presidential race. When he announced he was getting back into the Senate race last week, Rubio cited the prospect of a Trump presidency as one of the reasons. On CBS's "Face The Nation," Rubio said he wants to make sure the Senate acts as a check on the president, whoever it is.
(SOUNDBITE OF TV SHOW, "FACE THE NATION")
MARCO RUBIO: And that means the president of our own party. And if we agree on something, we need to work together with that president. And if we disagree on it, we need to be willing to stand up to the presidency, even if they're of your own party.
ALLEN: But there's a potential problem for Rubio. Among core Republican voters in Florida, the people most likely to cast ballots in the state's August primary, Trump is popular. Barry Jollett is the vice chairman of the Republican Party in Charlotte County on Florida's Gulf Coast.
BARRY JOLLETT: In Charlotte County, this is going to be Trump country, and I don't think any other issues are going to matter.
CARLOS BERUFF: Carlos Beruff, how are you?
LEAH LIPKE: Hi, Leah Lipke.
UNIDENTIFIED MAN: Hi. I know this lady.
ALLEN: For months now, Carlos Beruff has been visiting Republican gatherings like this one in all of the state's 67 counties. Beruff is a homebuilder, a Cuban-American with close ties to Florida's Republican governor, Rick Scott. In his speech in Charlotte County, Beruff talked about his distrust of political insiders, especially in Washington.
BERUFF: They don't represent you. And if you don't start breaking through that log jam of people who don't give a darn about us, we're never going to take our government back.
ALLEN: Six years ago, another Republican ran for the Senate in Florida as an outsider. Once elected, Rubio disappointed many conservatives, especially with his work as part of the Senate's gang of eight on an immigration reform plan. Here's Beruff's assessment.
BERUFF: Unfortunately, he drank the water, the Kool-Aid in Washington and became one of the establishment instead of a change agent. As I tell people all the time, if I get to Washington I'll bring my own water supply.
ALLEN: Unlike Rubio, Beruff likes Donald Trump and has endorsed him. He's hoping his support for Trump will help him break through with Republican voters.
BERUFF: Donald Trump is going to need help to move an agenda. Do we agree on everything? No but like I tell people, I don't agree with my wife on everything either (laughter). But at the end of the day, I think we agree on many things.
ALLEN: Rubio re-entered the race at the urging of party leaders worried about losing a seat that may help determine which party wins control of the Senate in the fall. But even among some of his supporters, Rubio's recent political calculations have seemed a little too calculated. Clifford Pierce, a Republican activist in Florida's Desoto County, voted for Rubio six years ago. This time, Pierce says, he's supporting Beruff.
CLIFFORD PIERCE: You can't run as an outsider forever. I mean, sooner or later you have to have some accomplishments to stand on. And I don't know that Rubio has those accomplishments.
ALLEN: Like Trump, up to now Beruff has largely self-funded his campaign, spending close to $5 million. He's reportedly told staff he will put in $10 to $15 million more. Before Rubio got back into the race, Beruff was the front-runner for the Republican nomination. Now as a well-known incumbent, polls show Rubio has a 50 or 60-point lead. And Beruff has a steep challenge with less than two months to the primary. Greg Allen, NPR News, Miami. Transcript provided by NPR, Copyright NPR.