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(CNN) —  

Florida Gov. Rick Scott demanded Friday that Democratic Sen. Bill Nelson provide proof of his claim that Russians had penetrated some counties’ voting systems.

Scott, the Republican trying to unseat Nelson in one of the nation’s marquee Senate races this fall, blasted Nelson at a campaign event in Tampa, saying he had made a “very serious charge” but “provided no proof.”

Nelson’s claim has drawn scrutiny from state officials, who have pushed back in recent days. The dispute takes place amid growing concern about Russian interference in US elections and in particular the security of the country’s election infrastructure.

“Either Bill Nelson knows of crucial information the federal government is withholding from Florida elections officials, or he is simply making things up,” Scott said, according to a prepared version of his remarks shared by his campaign.

“Did Nelson illegally release some classified information? Or did he make this charge of Russian penetration up?” Scott said.

The GOP governor’s criticism comes two days after Nelson told the Tampa Bay Times that Russians “already penetrated certain counties in the state and they now have free rein to move about.”

He told the newspaper that he had been “requested by the chairman and vice chairman of the Intelligence Committee to let the supervisors of election in Florida know that the Russians are in their records.”

He had made similar claims a day earlier in Tallahassee, but had declined to elaborate. “That’s classified,” he said then.

The Florida Department of State, which oversees elections there, said it had no evidence to support Nelson’s claims and that the senator should share any specific information he has with elections officials.

Nelson responded to Scott’s comments with a statement Friday that neither repeated his claims that Russians had already penetrated certain counties’ election systems nor offered evidence of his claims.

“I and several of my Senate colleagues are trying to make sure Florida officials are aware of the ongoing Russian threat so they take the steps necessary to safeguard our elections,” Nelson said in the statement issued by his Senate office. “It’s unfortunate that some Florida officials would try to use this issue for personal, political gain.”

Nelson’s office also sent reporters a letter that Senate Intelligence Chairman Richard Burr, R-North Carolina, sent Florida’s secretary of state on Friday. Burr’s letter does not directly address the veracity of Nelson’s claims, instead urging Florida elections officials to work with the FBI and the Department of Homeland Security.

It was the second time this week Nelson found himself under fire for problematic comments.

He has criticized Scott’s environmental record amid algae blooms and red tide in Florida, blaming Scott for asking the Environmental Protection Agency to delay the implementation of stricter water standards.

However, the Tampa Bay Times reported that Nelson had written a similar letter to the EPA at a similar time, in 2010. Nelson told the newspaper he did not recall that letter.

It led the Mitch McConnell-aligned Republican super PAC that invests millions of dollars in Senate races to accuse the 75-year-old Nelson of becoming senile.

“It’s time for Bill Nelson’s caretakers to keep better tabs on the senator’s whereabouts and public statements so that he is not embarrassed into admitting he’s no longer dealing from a full deck,” said Senate Leadership Fund spokesman Chris Pack.

Nelson campaign spokeswoman Carlie Waibel, responding to the Senate Leadership Fund, said, “It’s pathetic and shameful that the super PAC of the leader of the US Senate would engage in this kind of slanderous, dirty politics.”

The super PAC has attacked Nelson along similar lines before, including last month, when the senator changed his tune about whether he was willing to consider Supreme Court nominee Brett Kavanaugh.

“Bill Nelson is losing it and needs to retire,” Pack said then. “Or his staff should stop leaving the poor man unsupervised.”