The hotly contested U.S. Senate race has tightened even further.

As of 4:30 p.m. Thursday, Republican challenger Gov. Rick Scott was leading incumbent Democrat Bill Nelson by just 17,344 votes out of more than 8.1 million cast in the election.

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That's within the 0.25 point margin that dictates a hand recount of ballots that showed either no vote or more than one vote cast in the Senate race.

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"It's a jump ball," said Marc Elias, the lawyer overseeing the recount effort for Nelson.

Election officials in the Democratic stronghold of Broward County are still counting absentee ballots and early-voting ballots. In Palm Beach County, which also tilts Democratic, officials also are still counting absentee ballots.

Only a handful of the state's 67 counties have finished counting provisional ballots. Overseas mail ballots can continue to be received and counted until Nov. 16, as long as they were postmarked no later than Election Day.

Elias said it is possible Nelson will actually be leading the race by Saturday, when each of the counties have to submit their preliminary unofficial totals to the state. It is that first tally that dictates whether a recount is required.

But whatever that vote total shows Saturday, Elias said he expects the final results will show Nelson the winner. He points to an unusually high number of ballots in Broward County showing no votes in the Senate race. He believes that was caused by a technical error of some sort and that a recount will correct that.

So far, Nelson has captured 69 percent of the vote in Broward.

"I firmly believe at the end of this process, Sen. Nelson is going to prevail," Elias said.

"Let's be clear: When Elias says 'win,' he means 'steal,'" the Scott campaign said in a statement emailed to reporters.

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The Nelson campaign said it was deploying observers to all 67 Supervisor of Elections offices statewide as official there complete their canvassing of votes.

The canvassing boards — made up of the supervisor of elections, a county court judge and county commission chair — examine provisional ballots and outstanding absentee ballots to determine if they should be counted.

Particular attention will be paid to provisional ballots, which are cast when a person's right to vote can't be established at a polling place. A person who doesn't present a valid picture ID, for instance, would cast a provisional ballot.

Voters have two days after an election to provide additional proof that they were eligible to vote, such as presenting an ID at the election office. But voters don't necessarily need to do anything more to have a provisional ballot counted. For instance, if the canvassing board decides the signature provided with the provisional ballot matches the voter's signature on record, it can opt to count the ballot.

Still, both parties were telling members of their parties that had cast a provisional ballot to head to their local elections office to offer proof of their eligibility to vote.

Nobody seems to know how many votes are left to be counted.

The New York Times estimated as of Wednesday that there will still 113,000 votes to be counted. But that was based on a statistical analysis of turnout trends, not a tally of actual ballots collected by election officials.

State law mandates a machine recount of ballots in any race where the preliminary tally shows a margin of victory within 0.5 percentage points. In a machine recount, election officials run ballots through high-speed tabulation machines to double-check the numbers reported by precinct-level voting machines on election day. Those machines are tested for accuracy just before the recount and the results of the recount are presumed correct.

Machine recounts would take place early next week at election offices in each county.

If the machine recount shows a margin of victory of less than 0.25 percent, election officials then would begin a hand recount.

Any manual recount doesn't mean that election officials would look at each of the more than 8 million ballots cast in the state. Instead, they would be examining ballots that tabulation machines said had "overvotes" or "undervotes" in the contested race.

An overvote occurs when the tabulating machine believes a voter cast more than one vote in a certain race. In some cases, voters do fill out more than one bubble for a race. Such "true" overvotes are not counted.

But sometimes a stray pen mark or other voter error can cause the tabulation machines to register an overvote. In such cases, where the voter intent is clear from the ballot, the vote is counted.

"You are trying to figure if it was an intentional overvote, or if you can divine the intention of the voter," said George Levesque, an attorney who works on election matters at Gray Robinson's Tallahassee office.

Similarly, an undervote is when a tabulation machine says no vote was registered in a race. Voters sometimes decide not to vote in certain races, perhaps because they feel they don't know enough about the candidates.

But such true undervotes are more common in down-ballot races, not high-profile races like a U.S. Senate election. And, as with overvotes, sometimes they are caused by voters improperly filling out ballots. If the intent of the voter is clear from an examination of the ballot, the vote is counted.

Nelson's lawyer Elias points to an unusually high number of undervotes in the Senate race in Broward as particularly suspicious.

The county's current tally shows 676,706 votes cast in the Senate race compared with 701,469 in the governors race. Even down-ballot races such as the attorney general, chief financial officer and agriculture commissioner drew more votes than the Senate race.

“That’s just not plausible,” Elias said.

He believes any undercount was likely caused by a technical glitch in the tabulating machines. He notes that the undercount wasn't consistent throughout the county but only happened in certain voting districts.

If that's the case, a machine recount would rectify that. That could mean a huge vote swing in a county where Nelson otherwise captured seven out of every ten votes cast.

“I think we are going to see significant increase,” Elias said.

One wild-card still looms in the race: Overseas military ballots.

Absentee ballots from military members and dependents stationed overseas can be accepted until Nov. 16 as long as they were postmarked no later than Nov. 6. Conventional wisdom holds that military votes tend to break for Republican candidates. But until the Nov. 16 deadline comes, there is no way to tell how many overseas military ballots are outstanding.

Meanwhile, the Scott campaign said Nelson and Elias were resorting to "low tactics."

"It is sad and embarrassing that Bill Nelson would resort to these low tactics after the voters have clearly spoken," the campaign said in statement emailed to reporters. "Maybe next, he'll start ranting that Russians stole the election from him."

Contact McCarthy at 321-752-5018

or jmccarthy@floridatoday.com