Where the Florida Senate Race Stands Ahead of Tonight's Debate

— -- For months, as he eyed the White House, Marco Rubio promised voters he was done with the Senate.

“You don’t run for president with some eject button [if it doesn’t work out],” the freshman Florida senator told ABC’s Chief White House Correspondent Jonathan Karl in 2014.

Now, following his failed attempt to emerge from the 17-candidate field in the Republican presidential primary, Rubio finds himself in the thick of a re-election fight for his political career.

The Florida Republican announced his re-election bid shortly before the filing deadline in June, citing the Pulse nightclub shooting and the need to serve as a check on the next president, be it Hillary Clinton or Donald Trump.

The move cleared the field of lesser GOP Senate candidates, transforming a contest Democrats expected to win into an expensive battle that could decide control of the Senate.

The man standing between Rubio and another term in the Senate is Rep. Patrick Murphy, a moderate who defeated progressive Rep. Alan Grayson in a nasty primary with the help of endorsements from party leaders like President Barack Obama and Vice President Joe Biden.

While he’s struggled to compete with Rubio’s name recognition, the well-funded Murphy has stayed competitive by relentlessly tying the Florida senator to Donald Trump, whom Rubio ridiculed in the primary race but continues to support in the general election.

Republicans, in turn, have turned their sights on Murphy’s resume. A WFOR-TV investigation of Murphy found that the congressman worked as a CPA at Deloitte -- an accomplishment he has touted on the campaign trail -- for less than a year after obtaining his license in another state.

As for his small business -- an environmental cleanup company that worked in the Gulf of Mexico after the Deepwater Horizon oil spill -- it appears to be a unit of his father’s billion-dollar South Florida construction empire.

On policy, Murphy and Rubio have differing visions for how to address problems with Obamacare, with each sticking to their respective party lines. Rubio has called for a complete repeal of Obamacare and an institution of a tax credit to pay for insurance, while Murphy has argued that Obamacare protects people with pre-existing conditions the state’s young citizens seeking to stay on their parents’ plans.

Murphy has also supported President Obama’s efforts to develop relations with Cuba, an issue of focus given South Florida’s large Cuban-American population. Conversely, Rubio, the son of Cuban immigrants, pledged during his presidential campaign to roll back relations between the countries and said the trade embargo against Cuba could have been used to force democratic concessions from the communist government.

Recent polls show Rubio out performing Donald Trump in Florida, but with recent polling -- including a Quinnipiac poll released today -- showing Trump fading behind Clinton in the battleground state, Rubio could see his own numbers take a hit as a result. An NBC/Wall Street Journal/Marist poll from October 9 showed the race between Rubio and Murphy within the margin of error.

ABC's Senate race ratings classify Florida as "Leans Republican" ahead of the debate.

Tonight, the two Senate candidates face off in an hour-long debate at the University of Central Florida beginning at 7 p.m.