MIAMI, Florida — Now that Sen. Marco Rubio (R-FL) has officially thrown his hat in the presidential ring, the chatter is heating up about who among the many Republican contenders might run for his Senate seat.

Rubio has insisted that he is not running for re-election, opening up the field to those who would have never considered challenging him. And with the perceived frontrunnner, Florida Chief Financial Officer Jeff Atwater, declining to run, the field is now wide open.

Florida law does not allow Rubio to run for both president and Senate on the same ballot — an option that is available under Kentucky law for Sen. Rand Paul (R-KY), which he seems to be pursuing — but Rubio could certainly delay making the decision until closer to the May 2016 filing date for Senate.

In an appearance on Sean Hannity’s Fox News Channel show Monday evening, Hannity asked Rubio to confirm that it was set in stone that he would not be running for re-election to the Senate. Rubio was adamant that he had chosen his path and would step aside for a newcomer to succeed him in the Senate. “I’m running for President of the United States, and that’s what my campaign is about,” said Rubio.

“No fall back?” asked Hannity.

“I don’t have a Plan B to pivot back to the Senate race,” replied Rubio. “I intend to be the nominee. And that’s why I think it’s important for us to have a strong candidate in Florida who’s out there working now. If I went around talking about how I would pivot back to the Senate race if things didn’t work out, our best candidates may not run.”

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As Breitbart News reported, Florida’s Chief Financial Officer Jeff Atwater had been planning to run in the 2016 Senate race but changed his mind a few days ago, citing family reasons.

Among the other Republicans rumored to be considering running for Rubio’s Senate seat are Lieutenant Governor Carlos Lopez-Cantera, Rep. Ron DeSantis, Rep. Tom Rooney, Rep. Vern Buchanan, former Florida Speaker of the House Will Weatherford, and former Sen. George LeMieux.

Former Congressman Allen West had a strong showing in early polls but has become a Texas resident since leaving Congress and has publicly said that he is not interested in moving back for a political campaign.

Attorney General Pam Bondi, on the other hand, had previously said that she was focused on her job as AG and not interested in running for Senate. However, a source close to Bondi told Breitbart News that she is asking her allies to “stay on the sidelines for now” and not jump behind another candidate quite yet.

LeMieux’s previous ties to former Governor — and former Republican — Charlie Crist would likely sink his chances with Republican primary voters. With little name recognition beyond being Crist’s “right hand man” as his longtime campaign adviser and chief of staff, LeMieux was appointed by Crist to serve the remainder of Sen. Mel Martinez’s term in 2009. The close association with Crist and support for moderate political positions ruined his appeal for the Republican primary voters. When he attempted a Senate run in 2012, LeMieux ended up dropping out before the primary election was even held.

DeSantis is officially “considering” running, according to a statement provided to Breitbart News:

Marco Rubio has done a great job in the U.S. Senate. His 2010 campaign inspired me to consider running for office, and I have no doubt that he will make a compelling candidate on the national stage. As it became clear that Senator Rubio was likely to run for President, I received encouragement to consider running for the Senate. Casey and I will use the next several weeks to discuss the race with our friends and supporters and will make a decision in short order.

The country is suffering from stagnation at home and indignities abroad. We need a new generation of leaders who will promote policies that will foster economic growth and alleviate the middle class squeeze, defend America’s national security against those who threaten our people, reform the culture of Washington, D.C., and reassert the constitutional principles that make our country unique. Whatever shape my future service takes, I look forward to doing my part to help get our country back on track.

Elected in 2012 with strong tea party support and conservative endorsements like Club for Growth and Heritage Action for America, plus an Ivy League education and distinguished military service record, DeSantis has the credentials to be a serious contender. He would also start the race with a head start on fundraising: DeSantis reportedly has $1 million cash-on-hand in his Congressional campaign account that could be rolled over to a Senate race.

Following the news that DeSantis was considering running, several conservative groups made public statements praising him and indicating they would be likely to support him if he entered the race.

In an interview with National Journal, Drew Ryun, political director of the Madison Project, called DeSantis “a marquee candidate for the conservative movement in the 2016 cycle” and predicted that “if he makes the decision to run, you’d definitely see the conservative groups coalesce around [him], much like we did in 2012 when he ran for the House.”

Likewise, Senate Conservatives Fund President Ken Cuccinelli called it “extremely important for conservatives in Florida to elect a principled leader” to replace Rubio. “We’re looking at all of the candidates,” said Cuccinelli, “but if Congressman Ron DeSantis decides to run, we will seriously consider supporting his campaign.” Club for Growth President David McIntosh described DeSantis as someone who “would make a terrific U.S. Senator” and praised his record so far in Congress. “Rep. DeSantis has been a stalwart supporter of limited government and pro-economic-growth fiscal policies in the House, as we expected him to be when the Club for Growth PAC supported his initial House candidacy in 2012,” said McIntosh.

The Citizens United Political Victory Fund issued perhaps the strongest statement from their President David N. Bossie. Praising DeSantis as “a principled constitutional conservative leader” who had lived up to his campaign promises, Bossie vowed that if DeSantis threw his hat in the ring, “CUPVF will be with him every step of the way and will dedicate substantial resources for this effort.”

Both Rooney and Lopez-Cantera were at Rubio’s announcement in Miami Monday, and both told Breitbart News they are focusing on their current jobs, but said it in a way that made it very clear that they are considering the Senate race.

Breitbart News spoke to Lopez-Cantera right before Rubio’s announcement speech. “Right now I’m focused on making sure that we get our tax cuts through the Legislature, get the highest level of education funding in the history of Florida and our college affordability plan passed,” he said. “Once we get past those things, there will be time for the campaigns and politics later.”

“My primary focus is on serving the people of Florida’s 17th district, but I have received strong, positive feedback from many supporters and constituents about taking up the mantle of common-sense conservatism from Marco in the Senate,” said Rooney in an email statement to Breitbart News Tuesday morning. “Over the next several days and weeks, I will give careful consideration to how I can best serve our state and advance the causes that are critical to the future of our country – providing for a strong defense, tackling our debt crisis, and growing our economy through free market principles. Preeminent in my decision is the impact that a Senate run would have on my family.” Rooney has officially endorsed Rubio for president.

The issue for Republicans, who just won back majority control of the Senate, is to keep Rubio’s seat in Republican hands. Previously, sources told Breitbart News that both the National Republican Senatorial Committee (NRSC) and the Republican Party of Florida (RPOF) had expressed concerns to Rubio about this, and the NRSC especially had urged Rubio to stay in the Senate.

Rubio running for re-election is obviously no longer an option, and Republicans are searching for someone who is both a strong campaigner and a strong fundraiser. With 2016 being a presidential election year, and Democrats expressing a willingness to devote major resources to a Senate campaign, the race will be far from a cakewalk.

Florida’s primary election date is set for August 30, 2016, so the Republican nominee for Senate will have just over two months to raise money and execute a campaign plan for the general election, which will fall on November 8.

The Sunshine State is a tougher place to run statewide than many other states, with ten media markets that often have very little overlap and a significantly more diverse electorate than nearly everywhere else in the country. To be competitive, a candidate must raise a lot of money quickly to have the resources to establish sufficient name recognition and counter any attacks from opponents. And even with adequate fundraising, not all candidates prove capable of assembling the campaign infrastructure needed to get-out-the-vote across the state.

Florida Republicans have seen a number of candidates crash and burn attempting to win statewide elections. Former governors Charlie Crist and Jeb Bush both failed in their first attempts to run statewide, Bush challenging Lawton Chiles (D) for Governor in 1994 and Crist challenging Senator Bob Graham (D) in 1998. Former Congressman Bill McCollum ran twice to represent Florida in the Senate, in 2000 and 2004, before running for Attorney General in 2006 and winning, then running for Governor in 2010 and losing the primary to Rick Scott.

Another recent example is former Congressman Connie Mack IV (R), who fell on his face attempting to defeat Senator Bill Nelson (D) in 2012. Mack had easily won re-election for years in his Southwest Florida Congressional District, buoyed by his family name — father Connie Mack III had represented Florida in the Senate and great-grandfather Connie Mack was a Baseball Hall of Famer — and a heavily Republican district.

However, by 2012, Mack’s father had been out of the Senate for over a decade and the inherited name recognition had considerably weakened, especially outside of Mack’s Congressional district. Nelson would nearly double Mack’s fundraising and go on to beat him by over one million votes,  55.2 percent to Mack’s 42.2 percent.

Follow Sarah Rumpf on Twitter @rumpfshaker.