Florida Republican Sen. Marco Rubio on Thursday charged that two Democrat-controlled counties in the state, in possible violation of election law, have been reporting a "slow drip" of tens of thousands of additional ballots favorable to several Democratic candidates for statewide office, including incumbent Sen. Bill Nelson.

Liberal lawyers, Rubio said in an extraordinary series of tweets that alleged incompetence if not outright complicity by Florida officials, are "descending on" the state in a calculated attempt to "change the results" and "try and steal" races for Senate and agriculture commissioner.

Rubio's explosive posts come as a recount now appears imminent in Florida's high-profile gubernatorial contest between Democrat Andrew Gillum and Republican Ron DeSantis, based on new vote totals -- even though Gillum has already conceded.

"Florida law requires counties report early voting & vote-by-mail within 30 minutes after polls close," Rubio wrote on Twitter. "43 hours after polls closed 2 Democrat strongholds #BrowardCounty & #PalmBeachCounty are still counting & refusing to disclose how many ballots they have left to count."

Florida statutes mandate that the "canvassing board shall report all early voting and all tabulated vote-by-mail results to the Department of State within 30 minutes after the polls close" and that "thereafter, the canvassing board shall report, with the exception of provisional ballot results, updated precinct election results to the department at least every 45 minutes until all results are completely reported."

"Democrat lawyers are descending on Florida. ... They aren’t here to make sure every vote is counted." — Sen. Marco Rubio, R-Fla.

The state code continues, "The supervisor of elections shall notify the department immediately of any circumstances that do not permit periodic updates as required."

Vote totals in several major races in the state are changing rapidly, on an irregular schedule and sometimes late into the evening. GOP Senate candidate Rick Scott, also Florida's governor, was ahead of Nelson by roughly one-fourth of one percentage point as of Thursday morning, according to The Tampa Bay Times. The paper also said the agriculture commissioner candidates were separated by only 0.06 points. That race flipped in favor of the Democratic candidate on Thursday afternoon.

Broward County reported that significantly more votes were received in the agriculture commissioner contest than the much higher-profile Senate election. An attorney for Nelson's campaign suggested a computer error might explain that anomaly.

In the closely watched gubernatorial race, DeSantis' held a narrow 0.52-percentage-point edge over Gillum as of Thursday morning, extremely close to the 0.5 percent threshold needed to trigger a machine recount. Gillum has conceded the race, although his decision is nonbinding.

But by Thursday afternoon, unofficial figures had DeSantis up by just 38,515 votes out of the more than 8 million cast -- a lead of just 0.47 percent, low enough to trigger a mandatory recount, according to The Tallahassee Democrat. No recount has yet been announced by Florida's secretary of state, and the first unofficial count is expected to be verified Saturday by Florida's secretary of state.

"On Tuesday night, the Gillum for Governor campaign operated with the best information available about the number of outstanding ballots left to count," Gillum's campaign said in a statement Thursday evening. "Since that time, it has become clear there are many more uncounted ballots than was originally reported. Our campaign, along with our attorney Barry Richard, is monitoring the situation closely and is ready for any outcome, including a state-mandated recount."

Without providing legally sufficient justification, Rubio said, Democrat-controlled Broward and Palm Beach counties on Wednesday afternoon continued to report new ballots, cutting into Scott's already-thin lead and flipping the state's agriculture commissioner race to Democrats. Provisional ballots cast by voters without proper identification or at the wrong location are factoring into the late results, and Florida officials have rejected campaigns' requests for the identity of those voters, citing federal and state law.

Palm Beach County Supervisor of Elections Susan Bucher told Fox News that Palm Beach is still counting about 2,000 mail-in ballots where voters circled or highlighted (by drawing an arrow pointing at the candidate’s name) their choice, instead of filling in the appropriate bubbles.

Under Florida law, the state elections department is allowed to determine voter intent. Bucher told Fox News that elections department staff is going through each of the 2,000 mail-in ballots, and where voter intent is determined, a worker is filling in a new ballot on behalf of the voter. Where voter intent cannot be determined, the ballot is sent to the canvassing board to undergo review.

Elections officials are also reviewing 1,500 military and overseas ballots which are still being counted. Florida law allows military servicemembers to mail or fax ballots in. A provisional vote report is due in Tallahassee on Saturday by noon.

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"#Broward election supervisors ongoing violation of #Florida law requiring timely reporting isn’t just annoying incompetence," Rubio wrote. "It has opened the door for lawyers to come here & try to steal a seat in the U.S. Senate & Florida Cabinet."

Broward county election officials did not immediately return Fox News' request for comment.

Rubio added: "Now democrat lawyers are descending on #Florida. They have been very clear they aren’t here to make sure every vote is counted. - They are here to change the results of election; & - #Broward is where they plan to do it."

Under Florida law, if the margin in any race hits 0.25 percent or lower, a manual recount of any ballots set aside from the machine recount will be ordered -- reminiscent of the scene in the 2000 presidential election, when the country was gripped by images of poll workers counting votes deciphering hanging chads by hand.

The Supreme Court voted by a 7-2 majority that Florida's recount in that race violated the Equal Protection Clause of the Constitution because each country and precinct adopted varying standards for how it would conduct the recount. By a 5-4 vote, the Supreme Court ended the recount on other grounds.

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Rubio made clear he has no confidence in the integrity of South Florida's election officials, suggesting a posssible GOP-led court challenge might be looming.

"A U.S. Senate seat & a statewide cabinet officer are now potentially in the hands of an elections supervisor with a history of incompetence & of blatant violations of state & federal laws," he wrote, linking to a Miami Herald article describing several scandals that have gripped Broward County’s Elections Department.

Earlier this year, a judge found that Broward Elections Supervisor Brenda Snipes had illegally destroyed ballots in a 2016 congressional contest, leading the governor's office to assign election monitors to supervise her.

“I think the problems are blown out of proportion,” Snipes said in October, in an interview with The Miami Herald. “Broward is nitpicked to the bone. Other places have the same problems, different problems. It’s just that they are not spotlighted like we are.”

Fox News' Ivonne Amor contributed to this report.