By Nandita Bose and Trevor Hunnicutt

GOLDEN BEACH, Fla. (Reuters) -President Joe Biden on Tuesday cast potential 2024 Republican rival Ron DeSantis as "Donald Trump incarnate," warning voters days before U.S. midterm elections that Republicans could take away their healthcare and retirement benefits.

"Charlie is running against Donald Trump incarnate," Biden said at a fundraising event for Charlie Crist, the Florida Democratic candidate for governor, who is running against DeSantis, the state's incumbent Republican governor.

The comments were Biden's sharpest yet against DeSantis, who is widely believed to be considering a presidential run in 2024, along with former President Trump. Biden, whose approval rating is hovering around 40%, has said he intends to run but not formally declared his candidacy.

With just seven days to go until elections that will determine whether Democrats retain control of Congress and the ability to pass Biden's agenda, the Democratic president made his first trip in office to Florida of a partisan nature. Biden lost the state to Trump in 2020.

The president's trip also included a campaign event for U.S. Representative Val Demings, a Democrat who is seeking a Senate seat held by Republican Marco Rubio, as well as a speech attacking Republican policies on Social Security, Medicare and prescription drugs.

Democratic strategists worry that Florida, a closely contested political theater for decades, is starting to decidedly lean Republican. They hope to push back against that trend by making a campaign issue out of purported risks to popular health and retirement benefit programs.

Florida has one the largest shares of senior citizens among U.S. states. One in five residents is over the age of 65.

"You've been paying into Social Security your whole life, you earned it, now these guys want to take it away," Biden said earlier to a sparse crowd at a South Florida community center. "Who in the hell do they think they are?"

Republican Senator Rick Scott of Florida has proposed requiring Congress to periodically pass legislation renewing government programs including Social Security and Medicare that are now guaranteed without such a renewal. But Scott has also said that he and other senior Republicans want to preserve those benefit programs.

Biden and DeSantis, meanwhile, have clashed over multiple issues including COVID-19 vaccines, abortion and LGBT rights.

Biden met DeSantis last month during a trip to the state to assess devastation from Hurricane Ian. They greeted each other warmly and stood shoulder to shoulder as they met with victims of the hurricane.

In recent weeks, the White House has lowered its earlier optimism about the midterm elections, and administration officials say they are now worried Democrats could lose control of both chambers of Congress. Several key state governor races are also contested.

Recent polls have shown Democrats, who once had comfortable leads in some Senate races, on a knife's edge ahead of the Nov. 8 election. Senate elections that were considered toss-ups between the two parties are now leaning Republican as high inflation persists.

Losing control of one or both houses of Congress would profoundly shape the next two years of Biden's presidency, with Republicans expected to block legislation on family leave, abortion, policing and other Biden priorities.

"With God's grace, we are going to win two more seats in the Senate," Biden told campaign donors during the event for Crist in Golden Beach.

The chamber is currently evenly divided between the parties and controlled by Democrats only because of Vice President Kamala Harris' tie-breaking vote.

The last time a Democrat won a presidential or Senate election in Florida was 2012. Republicans hold an advantage in registered voters in the state, 5.2 million versus 4.9 million. But unaffiliated voters total 3.9 million and represent an increasingly important part of the electorate.