The number of uncounted votes in Alaska's tightly fought U.S. Senate race grew by 21,000 between Wednesday and Friday -- and more than 5,000 of those were votes that hadn't been predicted in early accounts of the number of ballots outstanding.

After election night on Tuesday, incumbent Democratic U.S. Sen. Mark Begich trailed Republican challenger Dan Sullivan by 8,000 votes, or 3.6 percent, and both campaigns have been closely watching as state elections officials collect additional ballots cast by mail, or at more than 200 so-called "absentee in-person voting locations" around the state, where people could vote early.

More than 40,000 ballots will likely be counted starting Tuesday, though the number will probably climb even more before then. To win, Begich would have to reverse election night trends and win a substantial majority -- though his allies have pointed out that in the count following Election Day in 2008, Begich overcame a 3,000 vote deficit to Republican Ted Stevens and ultimately won by 4,000 votes.

The spike between Wednesday and Friday was a reflection of state elections officials' new accounting for more than 13,000 provisional ballots, 2,200 absentee ballots submitted by fax, mail or email, and some 5,200 ballots cast early at the in-person absentee voting locations across the state.

The provisional ballots -- typically cast by voters at the wrong polling place, a small proportion of which won't count in statewide races -- were anticipated in early predictions of the number of uncounted votes, as were the absentees sent by mail.

But the 5,200 early votes have not been included previously in accounts of the outstanding ballots. State elections officials said Friday that they don't know how many more early votes they expect to arrive, or the proportion of the more than 200 early voting locations from which ballots have been collected.

Those ballots come from more than 200 early voting locations across the state -- including 161 in rural Alaska, where results skewed heavily toward the incumbent Democratic candidate, Sen. Mark Begich, and where his campaign put a huge emphasis on registering and turning out voters.

More ballots are still expected from the early voting places, and those ballots are not included in state elections officials' current totals of by-mail, by-fax or by-email absentee ballots that voters requested but haven't yet returned. That number sits at 11,600.

Alaska Division of Elections Director Gail Fenumiai said in an email that the state does not track the number of outstanding ballots from the early voting locations, though she added that it does have procedures in place to ensure that all votes cast at the early voting locations are ultimately counted.

Fenumiai said in an email that the ballots from the early voting locations are sent in periodic batches over a 15-day voting period.

A spokesman for Begich's campaign declined to comment.

Ben Sparks, Sullivan's campaign manager, declined to comment Friday.

The two campaigns have sparred over the last few days about whether Begich should concede the race. Sullivan's campaign has pressed media outlets to call the race, while Begich's campaign has refused to concede, saying all votes should be counted.

Three Alaska Native groups issued a joint statement late Thursday saying that "the election isn't over until 'rural Alaska sings.' "

Jim Lottsfeldt, an Anchorage political consultant who ran the pro-Begich Put Alaska First PAC during the election, said he's talked to Begich, whose message about the outstanding ballots was, "The more they find, the better it is for him, mathematically."

While Begich is "totally upbeat," Lottsfeldt said, he's also "totally realistic."

"We talked about some plans if it doesn't work out -- what he might do," Lottsfeldt said.

Randy Ruedrich, a former chair of the Alaska Republican Party who's worked on the state's redistricting process, did his own review of the uncounted ballots.

"I believe that the outstanding ballots favor Senator Sullivan," he said in a phone interview.

A large number of the uncounted votes, Ruedrich said, come from areas that skew Republican, like the Mat-Su and the Kenai Peninsula. He also said doesn't think Republican Gov. Sean Parnell, who trails independent challenger Bill Walker by about 3,000 votes, should concede.

Vote counting will start up again Tuesday, though absentee ballots sent from outside the U.S. can be received through Nov. 19 as long as they were postmarked on Election Day.

The reason that many of the absentee votes aren't counted immediately is because state elections officials need to check them against registers used at individual precincts on Election Day, to ensure that people didn't try to vote twice.