Florida Governor Rick Scott speaks ahead of U.S. President Donald Trump announcing policy changes he is making toward Cuba at the Manuel Artime Theater in the Little Havana neighborhood on June 16, 2017 in Miami, Florida.

Florida's Republican Gov. Rick Scott announced he will run for Senate on Monday, kicking off a tough challenge for one of the chamber's most vulnerable Democrats, Sen. Bill Nelson.

In the second-term governor, the GOP gets a strong recruit for its efforts to hold on to its narrow 51-seat majority in the Senate. The 65-year-old Scott — a noted ally of President Donald Trump — will try to flip a Democratic seat in a state the president won in 2016.

Scott's entrance sets up what will likely prove to be one of the most expensive and contentious races of the 2018 midterms. Nelson, a generally popular official seeking his fourth term in the Senate, will hope opposition to Trump that has boosted Democratic candidates across the country will help him to hold off a governor whose approval rating has recently climbed.

Scott, who made a fortune in health care, won Florida's gubernatorial election in 2010 as a political newcomer. He cannot run for governor again due to term limits.

The governor has made job creation his top priority and message during his time in office. The push aligns him with Trump, whom Scott endorsed for president in March 2016.

"We're going to do the exact same thing we've done in Florida," Scott said in a Facebook video announcing his candidacy Monday. "We're going to turn around the national economy. We're going to make sure that Washington works for us."

In a separate video released by his campaign Monday, Scott cast himself as an outsider who "won't fit in in Washington," which is "full of politicians."

Scott tweet

Scott will have to balance an association with Trump against backlash to policies pursued by the president and GOP-controlled Congress. Trump himself urged Scott to run for Senate last year during a visit in September to view the damage from Hurricane Irma.

"I hope this man right here, Rick Scott, runs for the Senate," the president said at the time.

Asked by Politico on Sunday whether he considers himself a Trump Republican, Scott responded, "I consider myself Rick Scott. I don't consider myself any type of anything." He added: "I grew up poor. I believe in jobs."

Scott's bid for the Senate seat will force Senate Democrats and their outside backers to work harder to protect the seat. The party's resources could be stretched as Democrats and independents who caucus with them defend 26 seats in November, including 10 in states Trump won in 2016. Democrats will also try to compete for at least a pair of GOP-held Senate seats in Nevada and Arizona.

As Scott got ready to announce his bid Monday, Senate Democrats' campaign arm painted him as a self-serving politician. The Democratic Senatorial Campaign Committee criticized Scott's role in or response to recent tragedies such as the deaths of 14 nursing home residents after Hurricane Irma, the collapse of a bridge at Florida International University and the shooting massacre of 17 people at a Florida high school.

The organization noted that Scott's two bids for governor on 2010 and 2014 came during strong years for the GOP. The governor has "never run weighed down by an unpopular GOP president and Congress," which is "sparking a backlash against GOP candidates and deprives him of a traditional campaign message," the Democratic committee said.

Scott, who has an "A+" rating from the National Rifle Association, may have sensed the changing political tide as he backed new gun regulations earlier this year. Following the February shooting at Marjory Stoneman Douglas High School, he signed a law putting new restrictions on purchasing long guns and banning bump stocks, which effectively can make semi-automatic rifles automatic.

Scott's approval rating has climbed in recent months. A February Quinnipiac University poll found 49 percent of Florida voters approve of Scott, compared with 40 percent who disapprove. In the same survey, 42 percent of voters said they approved of Trump, while 54 percent said they disapprove.

Nelson had a 46 percent to 42 percent edge over Scott in the Senate contest, according to the poll. Forty-eight percent of respondents said they approved of Nelson, while 34 percent said they disapproved, according to the survey.

Asked about Scott's run later Monday, Nelson told reporters he always respects his opponents.

"I don't care who is an opponent. I always take them seriously, and I run like there's no tomorrow," he said. "And I think in this case a lot of the differences between the two of us are going to come out in the course of the campaign."