ORLANDO — Beginning what will be one of the country's most expensive and consequential midterm elections, Florida Gov. Rick Scott announced Monday he will challenge Democratic Sen. Bill Nelson, trying to recapture the outsider spirit that thrust the millionaire from political obscurity to two terms in office.

"I didn't fit into Tallahassee because I didn't play the insider games," Scott, 65, told a crowd at an Orlando construction company warehouse. "And guess what? I'm not going to fit into Washington, either."

At stake is not just who holds Florida's second Senate seat. The race is seen as a referendum on President Donald Trump, and it is crucial to Democrats' hopes of retaking the Senate this fall amid swelling grassroots enthusiasm.

Scott, a former health care executive, spent more than $80 million of his own fortune on his first two elections and will pose the toughest challenge three-term Sen. Nelson has faced, while forcing national Democrats to pour financial resources into a state Trump narrowly won in 2016.

Fact-checking Gov. Scott's announcement speech

With no notable primary opponents for either Scott or Nelson, the race escalates immediately with general election hype.

The battle got under way with an array of Democratic groups attacking Scott's record on the environment and economy and Republicans casting Nelson, 75, as a career politician lacking notable achievements.

"We shouldn't be sending the same type of people to Washington," Scott said in the sweaty warehouse, surrounded by wooden pallets and supporters fanning themselves with his campaign signs. "This concept of career politicians has got to stop."

Notably, Scott kicked off the campaign in the heart of the I-4 corridor, where statewide elections are won or lost. And he began his remarks with an appeal to the state's growing Puerto Rican population. He also said he'd advocate for term limits, a populist idea with no traction in Washington.

8 years ago today, I did something that everyone told me not to do – I announced that I was running for Governor. With the hard work of Floridians, we have come a long way, but our work is not done. That's why I have decided to run for the United States Senate. Let’s Get to Work! pic.twitter.com/zirHas1mzR — Rick Scott (@ScottforFlorida) April 9, 2018

Nelson sought to project confidence Monday and eagerly noted that Scott avoided talking about Trump, who has long encouraged his ally to enter the race.

"Go to the beach. Look out there, see if you see any oil rigs," the only statewide elected Democrat said from his office in Washington, responding to attacks on his record. He has accused Scott of flip-flopping on oil drilling.

In an interview on Fox News, Nelson noted how Facebook CEO Mark Zuckerberg sat down with him in advance of an appearance Tuesday before the Commerce Committee, of which Nelson is the ranking Democrat. "It takes time to work up through the seniority system, so I would think that that's particularly important to Florida."

While Scott did not mention Trump in his announcement speech, he invoked the president's winning message, promising to "fix" Washington and denouncing the "tired old thinking" in the nation's capital. "We gotta stop sending talkers to Washington. Let's send some doers to Washington," he said.

"Drain the swamp!" someone in the audience yelled.

Scott's kickoff tour included a rally in Fort Myers later Monday and a fundraising dinner at the St. Petersburg home of developer and former Ambassador Mel Sembler. On Tuesday, he'll hold events in Tampa and Hialeah.

Despite rising optimism among Democrats, who have won several special elections on the back of a charged-up base angry with Trump, Nelson faces a difficult challenge. He's one of 10 Democrats up for re-election in states Trump won.

Even if Nelson succeeds, his party may remain in the minority, as Democrats are in a tougher spot, having to defend 26 Senate seats vs. 9 for Republicans. The GOP now controls the Senate 51-49.

"Democrats have a great environment to run in, but the map is still very challenging," said Jennifer Duffy, an analyst with the nonpartisan Cook Political Report, which on Monday moved the Florida race from "lean Democrat" to "toss-up."

Recent polling has shown Nelson with a slight advantage, but both sides consider it very close, dependent not only on the rhythms of the campaign but the national mood and Trump's performance.

When Nelson first ran for office in 2000, spending topped out at $19.3 million. In the 2016 Florida Senate race won by Marco Rubio, spending hit $110.5 million, much of it coming from outside groups unbound in the era of super PACs and "dark money."

Given the resources needed to remain competitive and that Nelson hasn't been on the ballot in a while, Duffy argued that "in some ways, Nelson occupies the No. 1 spot right now" on vulnerability.

"The one thing he's got working for him now is Scott's never had to run in this kind of an environment," she said.

On Fox News on Monday, Scott avoided his first foreign policy question since declaring his campaign. When host Dana Perino asked him how the U.S. should respond to the recent gas attack in Syria, he responded, "I'd be listening to my military advisers."

Otherwise, Scott's script Monday was nearly identical to his 2010 campaign. He's reusing his "Let's Get To Work" slogan, and he's rehashing the same talking points about being an outsider running against career politicians.

In 2010, he told the Times, "There's going to be a clear choice between career politicians with their old ideas and stuck in the status quo, and a complete outsider with fresh ideas."

On Monday, he again denounced career politicians, telling the Fox host, "Their ideas are tired. Their ideas are old."

But if his campaign appears the same, it's because he's facing, for the third time, a politician who has spent decades in elected offices.

Scott came out of nowhere in 2010, riding the tea party wave to his first elected office. He was known primarily as the health care executive who oversaw massive fraud, and his company, Columbia/HCA, paid a record $1.7 billion in fines and pleaded guilty to 14 felonies.

But he found success running on an obsessive jobs platform at the height of the Great Recession, using his millions to eke out narrow wins in races for governor in 2010 and 2014. His finances could face new scrutiny this election, as federal law requires him to disclose more than Florida law does.

Scott has frequently generated controversy during his governorship, and he's never been considered an especially beloved or charismatic figure on the campaign trail.

For the past few months, Scott has been reaching out to the Puerto Rican community, and he made a clear play for that constituency on Monday. He was introduced at the rally by the territory's lieutenant governor, Luis G. Rivera-Marin, and Scott closed his speech in Spanish. Nelson too has targeted the Puerto Rican community.

Scott's message resonated with Steve Gumiela, a retired Land O'Lakes police officer wearing a red USA hat with "45" stitched into the side at Monday's rally.

"It started with Trump," he said, "and Scott will help finish it."