Voters in the US state of Georgia are returning to the polls to decide one of two unresolved Senate seats.
Neither incumbent Republican Senator Saxby Chambliss nor Democrat Jim Martin won a clear majority in the three-way first round vote.
A Democrat victory would help them towards the 60 seats they need to overcome any Republican "filibuster" attempts to block legislation.
The Democrats have 58 seats. Another vote in Minnesota is being recounted.
The Democrats won expanded majorities in both the House of Representatives and the Senate in the 4 November polls which saw their candidate, Barack Obama, elected president.
Republican 'firewall'
Voters in Georgia supported Republican candidate John McCain in the presidential election and Mr Chambliss, first elected as a senator for Georgia in 2002, had the lead in opinion polls going into the run-off vote.
Mr Chambliss promised voters he would act as a "firewall" against a Democratic-controlled Washington if he were elected.
Former Republican vice-presidential candidate Sarah Palin made several campaign appearances for Mr Chambliss, urging voters to begin the party's rebuilding.
"If we are to lead again, we have lots of hard work ahead of us. Let it begin here tomorrow in Georgia," she said at a rally in the town of Perry on Monday.
Mr Martin told voters he would work with President-elect Barack Obama to help him enact his agenda of change. He also promised economic help for the middle class.
Mr Obama recorded a radio advertisement for Mr Martin, but did not campaign in the state, despite a request. He did send 100 activists to Georgia to help get out the vote for Mr Martin.
Another Senate vote is being recounted in Minnesota, where Republican Senator Norm Coleman is locked in a tight race with Democrat Al Franken, a former comedian.
The Democrats already have 56 Senate seats plus two independents who have promised to vote with the Democrats.
Sixty seats or more in the 100-seat chamber would enable the Democrats to block Republican "filibustering" - attempts to delay or block the passage of legislation through a variety of procedural devices, such as excessively long speeches, tabling large numbers of amendments or raising spurious points of order.
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