Washington (CNN) President Donald Trump launched what opponents decried as a racially loaded attack Monday on Florida Democratic gubernatorial candidate Andrew Gillum, labeling him a "thief" without evidence and claiming that, as mayor of Tallahassee, Gillum oversees one of the country's "most corrupt cities."

The President's tweet comes just eight days before the election to decide who will lead the state: Gillum or his Republican rival, former Rep. Ron DeSantis. Early voting is already underway and Gillum, who is vying to become the state's first African-American governor, led by double digits in a recent CNN poll.

In Florida there is a choice between a Harvard/Yale educated man named @RonDeSantisFL who has been a great Congressman and will be a great Governor - and a Dem who is a thief and who is Mayor of poorly run Tallahassee, said to be one of the most corrupt cities in the Country!

Gillum responded less than an hour after Trump's attack and, like the last time they clashed, mocked the President for not engaging him more directly.

"On Twitter there is a choice between having the courage to @ the person you are trash talking, or not," Gillum wrote. "@realDonaldTrump is howling because he's weak. Florida, go vote today."

His opponent, DeSantis, has keyed in on Gillum's 2016 decision to accept a ticket to see the Broadway show "Hamilton" from a group that included an undercover FBI agent, according to text and email records released under subpoena last week, as evidence that an ongoing federal corruption probe in Tallahassee is closer to the mayor than has been reported. DeSantis has also questioned whether a trip Gillum took that year to Costa Rica with his wife, along with lobbyist and longtime friend Adam Corey and his associates, was appropriate. Gillum has said he paid his own way.

No one connected to Gillum or the investigation has been charged with a crime, and Gillum has repeatedly said the FBI has told him he is not a focus of the probe. There has been no suggestion of outright thievery, as suggested by Trump, even by Gillum's political opponents. The probe centers on whether developers successfully influenced city projects, although at one point an undercover agent infiltrated Gillum's inner circle and attended the trip to New York with Corey, Gillum and Gillum's brother.

Despite Gillum's assertions that he is not a target of the federal probe, White House press secretary Sarah Sanders claimed Gillum was under investigation by the FBI when asked at a news conference on Monday what the President meant when he used the word "thief" to describe the candidate.

"That individual's under FBI investigation. I would refer you to that," Sanders said. "I'm not going to get into specifics on that matter because it involves two candidates running for office and I'm not going to impact that due to the Hatch Act. I'll leave that to the President to make those kinds of statements."

Gillum's campaign dismissed Sanders' assertions as "desperate falsehoods" in a statement Monday.

"These desperate falsehoods make it clear the Trump Administration is more concerned with saving the failed gubernatorial campaign of its loyal lapdog Ron DeSantis, than it is with the truth," the statement said. "It's simply wrong to say Mayor Gillum is under FBI investigation—he's not under FBI investigation and the FBI has confirmed it. Conspiracy theories aren't true just because President Trump and Ron DeSantis pretend they are."

The President's comment earlier Monday sparked an instant backlash from an array of Democrats, including the group led by billionaire donor Tom Steyer, one of Gillum's top supporters, which described the language as "another racist dog-whistle meant to stir up his base because he knows DeSantis is behind."

DeSantis began the general election campaign by warning voters not to "monkey this up" by electing Gillum, a remark he subsequently denied had any racial component. DeSantis has also faced questions over his role as an administrator on a racist Facebook page, which a spokesman for him said the former lawmaker had been added to unwittingly and had since left.

Questioned last week at their second debate over his decision to speak at conferences hosted by David Horowitz, an anti-Muslim conservative political activist, DeSantis angrily rejected any suggestion he shared those views.

The exchange teed up Gillum, who shot back, "My grandmother used to say, 'A hit dog will holler,' and it hollered through this room," adding that while he wasn't calling DeSantis a racist, "I'm simply saying the racists believe he's a racist."

The records relating to Gillum's theater excursion in New York, which were subpoenaed by the state ethics commissions two weeks ago, according to the Tampa Bay Times, cast doubt on the explanation from Gillum's campaign that the mayor received the ticket from his brother, Marcus. At a debate last week, Gillum took responsibility for not asking "more questions" about the situation at the time.

"I was aware that Adam Corey and (an agent known as) Mike Miller arranged so that we could go and see the show. I arrived at the theater and received my ticket from my brother," Gillum said. "The problem that I have is that I should have asked more questions to make sure that everything that had transpired was aboveboard."

Gillum had also addressed the controversy earlier last week at a CNN debate with DeSantis.

"Did you pay for the 'Hamilton' ticket, or did the undercover FBI agent pay for the 'Hamilton' ticket?" DeSantis had asked.

Gillum never directly answered, but said he had -- in general terms -- always paid his own way during his political career.

"We all have friends that sometimes let us down," Gillum said of Corey, while also suggesting DeSantis was using the issue as a not-so-subtle way of trying to provoke a racial backlash in a campaign that could end with the election of Florida's first African-American governor.

"I'm a hardworking person," Gillum said. "I know that may not fit your description of what you think people like me do."