Mr. Nelson had hoped to pick up as many as 9,800 votes in Broward County as a result of a large number of so-called undervote ballots — those in which a voter cast a vote, for example, in the governor’s race, but not in the Senate race.

Broward County was unusual in that it had reported more than 30,400 of the undercount ballots. If they were not miscounted, then the most likely explanation was that they were in fact left blank, possibly because of the way the ballot was designed, with the Senate choices on the bottom left corner of the page, tucked beneath the voter instructions. Many have said it made the Senate race easy to miss.

Elections officials have not yet officially reported the new tallies, and in Broward, they said they would not do so until Sunday. But with the count incomplete, the Democratic official, who was not authorized to discuss the campaign’s deliberations publicly, said it appeared that while Mr. Nelson had picked up some votes, they were not enough to put him over the top.

“It looks like this is the end of the line for the Nelson campaign. He was really relying on this undercount being due to machine error which, if that were the case, would have revealed more Senate votes,” said Matthew Isbell, a Democratic data consultant from South Florida. “Considering how blue Broward is, it would have netted him thousands of additional votes. Without Broward, there is no obvious way he can make the votes up.”

Mr. Scott’s campaign quickly came to a similar conclusion. “With the hand recount concluding in most counties across the state showing no significant change in the margin, it’s time for Bill Nelson to face reality and concede,” Chris Hartline, a spokesman for the Scott campaign, said in a statement.