Sen. Bill Nelson, left, and Gov. Rick Scott Bill Clark | CQ Roll Call | Getty Images; Getty Images

Gov. Rick Scott jumped into Florida's Senate race on Monday, entering a battle with Democratic incumbent Sen. Bill Nelson that will likely be determined by how many millions of dollars each candidate spends come Election Day. One plugged-in Republican strategist told CNBC that it "will be one of the most expensive Senate races ever run." Both Scott and Nelson are likely to rack up huge amounts of donations, said the strategist, Alex Conant, former spokesman for Florida Republican Sen. Marco Rubio's 2016 presidential campaign. But look for outside donor groups to ante up big time, he added. "Money always matters, but both candidates will be very well-funded," said Conant, currently a partner at consulting firm Firehouse Strategies. "I would expect all the big outside groups on both sides to engage in this race, especially if it remains close until the end."

Where Scott could get support

Scott already has one money advantage coming into the election: He was the chairman of New Republican PAC, a political action committee that claims to be dedicated to rebranding the GOP. The group has already been active in the 2018 election cycle, having raised $1.7 million so far this year. The PAC's biggest donor, Comprehensive Care Group, a health-care center based in South Florida, contributed $250,000, according to Federal Election Commission filings reviewed by CNBC. The group also has the backing of the founder of private equity firm Energy Capital Partners, Douglas Kimmelman, who has given more than $50,000. The PAC currently has just more than $899,000 on hand, a good start for Scott if the PAC chooses to back him for Senate. Scott also could get the blessing of an organization that follows the beat of President Donald Trump's drum, America First Action Super PAC. The group's leader, Brian O. Walsh, recently told CNBC that the PAC did not rule out backing the Florida governor's Senate bid. "It's a terrific opportunity and an aggressive race," Walsh said. "What it means for us and what we will engage in, we will determine that throughout the year." America First Action has the financial support of Republican megadonors from across the United States, including real estate tycoon Geoffrey Palmer, who has coughed up $2 million to the PAC, and Ronald Weiser, chairman of the Michigan Republican Party, who contributed $200,000. Even before Scott's announcement, one powerful PAC has been on the offensive against Nelson since he voted against the GOP tax reform bill passed last year. The Senate Leadership Fund, a super PAC led by Senate Majority Leader Mitch McConnell's former chief of staff, Steven Law, has ripped the Florida Democrat for months, including with a website dedicated to attacking him. On the site, titled Back Bench Bill, a digital attack ad pops up and calls for Democratic lawmakers to lose their jobs over not voting for the tax bill. A spokesman for the PAC declined to comment about its future strategy with Scott's inclusion in the race. A spokesperson for Scott did not return a request for comment.

Nelson's war chest