The seat is one of at least five controlled by Republicans that Democrats want to win in November to take outright control of the chamber -- something essential to advance the agenda of a President Hillary Clinton or thwart the policies and nominees of a President Donald Trump.
The Senate Majority PAC, a political action committee associated with Senate Democratic Leader Harry Reid, has reserved $10.5 million in television ad time beginning in September and running through Election Day, the group said. The hefty figure is an indication Democrats are serious about winning the seat, which is in one of the most costly media markets in the country.
Last month, the Democratic Senatorial Campaign Committee also reserved $10 million for the race.
There are currently four Republicans and two Democrats running for the seat with a primary set for August 30 that will narrow the field. On the Republican side, Rep. David Jolly, who is considered centrist, is squaring off against tea party-backed Rep. Ron DeSantis, Florida t. Gov. Carlos Lopez-Cantera, who Rubio supports, and businessman Carlos Beruff.
"Republicans are going to nominate a far right, tea party candidate who is out of touch with Florida's middle-class. We're going to be ready to hold their eventual nominee accountable," said Shripal Shah, a spokesman for the super PAC.
On the Democratic side, Rep. Patrick Murphy, an establishment favorite, is squaring off against Rep. Alan Grayson, a populist liberal who has a fraught relationship with Reid.
A recent Quinnipiac University poll found the race too close to call although Murphy held a slight advantage over the Republicans in the field.
In an unusual move for a party leader, Reid endorsed Murphy earlier this year and called for Grayson, to get out of the race. Grayson, who is in his second term, has been dogged by ethics charges related to a hedge fund he ran while in Congress, which Reid cited when he called on him to go.
Grayson and Reid squared off at a recent closed-door meeting of congressional Democrats where Grayson challenged Reid to provide evidence of his alleged wrongdoing. Reid told Grayson, "I want you to lose."
It's unclear what Reid's super PAC will do if Grayson wins the primary.
The news of the Democratic infusion of cash comes as the Miami Herald reported
Monday that Murphy embellished his academic achievements, saying he earned a single degree despite repeatedly claiming he earned dual degrees. The campaign told the newspaper the mistake was an "inadvertent error."
The Senate Majority PAC has already reserved $7.5 million in New Hampshire and $9.5 million in Ohio as it tries to unseat vulnerable Republicans Kelly Ayotte and Rob Portman respectively. It has also reserved $8.5 million for ads in Nevada in an effort to hold onto the seat of Reid, who is retiring.
Republicans, who have a 54 to 46 advantage in the Senate, are defending 24 seats in November while Democrats are protecting just 10.