The final votes are being counted in Alaska's knife-edge Senate race between incumbent Republican Ted Stevens and Democrat Mark Begich, BBC reported.
With 24,000 votes still to be counted, Mr Begich leads by just 1,022 votes.
Meanwhile, the close senate race in Minnesota is set to go to a recount, and campaigning is under way ahead of a runoff vote in Georgia on 2 December.
The Democratic Party has already gained a handful of Senate seats and could yet end up with 60 out of 100.
This would be enough to prevent Republicans filibustering - or "talking out" a piece of legislation, so that it fails to pass within the time available.
The Alaska senate race was plunged into controversy when Mr Stevens, the longest-serving Republican in the Senate, was convicted of lying about gifts he had received from an oil company.
Mr Stevens is appealing against the verdict, but his fellow senators may vote to expel him from the chamber if he wins re-election.
Mr Begich is expected to stay in the lead, because the uncounted votes all come from regions in which he has performed well.
In Minnesota, Democrat Al Franken, a well-known comedian, trails incumbent Republican Norm Coleman by just 206 votes, or 0.007% of the vote.
According to Minnesota state law, a winning margin of less than 0.5% of the vote triggers an automatic manual recount. The decision is expected to be taken on Tuesday, with the recount starting on Wednesday.
When ballots are counted by hand, candidates can often pick up votes, because human counters register votes that counting machines ignore.
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 three-point lead, with 49% to Mr Martin's 46%.
Former president Bill Clinton is due to campaign for Mr Martin on Wednesday