- Democratic Sen. Bill Nelson aims to defend his seat amid a strong challenge from Republican Gov. Rick Scott.
- Scott, who is a former executive of a health care company and is worth at least $250 million, had piled a staggering $20.7 million of his own money into the race by early August.
- The candidates' campaigns and outside groups backing them have already spent heavily on attacks, even before Tuesday's primary elections.
Florida voters have not even chosen their nominees for a pivotal Senate contest yet, but the race between Democratic Sen. Bill Nelson and Republican Gov. Rick Scott is already the most expensive of the 2018 midterms.
The bitter general election essentially started when Scott declared his candidacy in April. It will formally begin after Tuesday, when Democratic and Republican primary voters are expected to overwhelmingly choose Nelson and Scott, respectively. (Update: NBC News projected both Nelson and Scott to win Tuesday night.)
The candidates and outside organizations supporting them have already spent heavily in Florida, where Republicans have one of their best opportunities to pick up a Senate seat this year. The campaigns of Nelson and Scott, plus independent groups trying to influence the race, have already shelled out more than $50 million, according to the Center for Responsive Politics.
It is currently the most expensive midterm contest in the country, when outside spending and campaign expenditures are added up, ahead of the highly competitive Missouri Senate race between vulnerable incumbent Democrat Sen. Claire McCaskill and Missouri's Republican attorney general, Josh Hawley.
The spending underscores how important the Florida race is in the fight for control of the Senate. If Scott wins, he gives a major boost to GOP efforts to expand the party's 51-49 seat majority in the chamber. The party balance in the Senate has proved critical on numerous tight votes during the Trump administration, including now as Republicans push to confirm Brett Kavanaugh, the president's pick to replace retired Supreme Court Justice Anthony Kennedy.
Scott is expected to give Nelson a fight for his political life as the understated Democrat seeks a fourth term in the Senate. An average of recent polls finds the governor leading the likely general election matchup by about 1.5 percentage points, according to RealClearPolitics.
While polls have trended in Scott's direction, Nelson should hardly be counted out, according to Michael McDonald, an associate professor of political science at the University of Florida. He said Scott has gotten a boost from his early media spending, but he expects Nelson and outside supporters to try to reverse the tide with their own ad push.
"There's ample time for Nelson to get up on the airwaves in an aggressive manner to turn around these poll numbers," he said.
Scott's personal wealth has helped the governor's cause in Florida. By earlier this month, his campaign had raised $31.1 million and spent $27.8 million. Scott, who is a former executive of a health care company and is worth at least $250 million, had piled a staggering $20.7 million of his own money into the race at that point.
Nelson's campaign had taken in $19.7 million and spent $6.1 million as of earlier this month.
At the same time, the vast majority of the outside money flooding into the race has focused on Nelson. Independent groups have spent $7.4 million to boost Nelson, versus $6.2 million opposing him. Outside organizations have spent a total of $1.8 million both for and against Scott.
The Senate Majority PAC, a so-called super PAC dedicated to aiding Democratic senators, has put more than $6 million into the race. New Republican PAC, another super PAC designed to help Scott get elected, has shelled out $5.3 million.
Attack ads started well before the primary in Florida, one of the 10 states that Trump won where a Democratic senator faces re-election this year. While the president only narrowly won Florida, both Scott's statewide name recognition and early spending push help his chances.
Pro-Scott forces have in part criticized the 75-year-old Nelson as a career politician who does not have the energy for the office. Nelson has also had to play defense over a questionable claim that Russians accessed Florida's election records, which federal officials have denied. The Scott campaign released an ad characterizing Nelson as "confused" over the accusations.
Nelson campaign spokesman Dan McLaughlin said Scott has tried to "create a political controversy where none exists" to distract from his own record.
Nelson's campaign and his outside allies have targeted Scott frequently with accusations that his investments got a boost from decisions the governor made while in office. Financial disclosures filed last month show that Scott and his wife have a stake in companies affected by state government policy, according to The Associated Press.
Scott campaign spokesman Chris Hartline says the governor's assets are in a blind trust managed by a third party. He said Scott "has no control of what is bought or sold."
National issues such as health care and jobs will factor into the Florida race. But Nelson and Scott have sparred over a handful of local issues that appear particularly important.
The state is dealing with toxic algal blooms, which are affecting the state's coasts as well as water inland. Nelson has said the algae "threatens our health, our economy and our way of life" and contended that Scott exacerbated the problem by weakening state environmental agencies.
Scott spokesman Hartline contended that Nelson, as part of the federal government, has neglected the issue because pollution at the state's Lake Okeechobee has contributed to the algal blooms. The federal government manages a dike at the lake.
Nelson and Florida's other senator, Republican Marco Rubio, have introduced a bill that would make a federal department come up with a plan to control the blooms.
The University of Florida's McDonald identified algal blooms as one area where Nelson could make up major ground on Scott.
One national issue has hit close to home in Florida: gun control. The state endured its latest highly publicized shooting on Sunday, when three people, including the gunman, died and 11 others were wounded at a video game tournament in Jacksonville.
In February, 17 people died in an attack at a high school in Parkland. In 2016, 50 people — including the shooter — died in an attack on a gay nightclub in Orlando.
Nelson has supported tougher background checks and a ban on some assault-style rifles. Scott, though a champion of gun rights as governor, signed a bill earlier this year that in part raised the age for buying rifles to 21 following the Parkland shooting.
While activists have tried to engage more young voters in Florida to vote for pro-gun control candidates, it is unclear yet if they will vote in higher numbers than they usually do.
—Graphics by CNBC's John Schoen