The recount could see Mr Coleman (left) losing to Mr Franken
Democrat Al Franken has gained votes in his Minnesota Senate race against Republican Norm Coleman as officials conduct a statewide manual recount.
At the end of the first count, Mr Coleman led Mr Franken by 215 votes, but with nearly half of the votes recounted, his lead had slipped to 145.
Officials have until 5 December to recount some 2.9 million votes.
The manual recount was automatically triggered because Mr Coleman's winning margin was less than 0.5% of the vote.
When ballots are counted by hand, candidates can often pick up votes, because human counters register votes that counting machines ignore.
And because incorrectly rejected ballots tend to have been cast by groups of voters - like the elderly and ethnic minorities - which usually vote Democratic, experts predict that Mr Franken will pick up more votes than Mr Coleman in the recount.
Apart from Minnesota, one other Senate election result remains outstanding.
REMAINING SENATE RACES
Minnesota: Manual recount ongoing
Georgia: Run-off scheduled for 2 December
In Georgia, because no candidate won more than 50% of the vote, a run-off between the top two candidates was automatically triggered.
Incumbent Republican Saxby Chambliss faces Democratic challenger Jim Martin in the 2 December vote.
A recent opinion poll from Research 2000 gave Mr Chambliss a six-point lead, with 51% to Mr Martin's 45%.
If Mr Franken overturns Mr Coleman's lead in Minnesota, and if Mr Martin is victorious in Georgia, then the Democrats will have 60 seats in the Senate, which could help them to frustrate attempts by Republicans to block legislation.
Sixty votes are needed to overturn a "filibuster" - tactic used by the Senate minority to prevent legislation from coming to a vote.