Republicans are confident they will take control of the US Senate in this week's congressional elections, but Americans may still have to wait until December or even 2015 to learn who controls the chamber.
Recent polls show Republicans pulling ahead of President Barack Obama's Democrats in the battle for power in Washington, despite races remaining close in Alaska and North Carolina.
Democrats currently hold a 10-seat Senate advantage, with 55 members to 45. If Republicans take a net six seats, Obama will spend his last two years in office facing a hostile Congress.
"We intend to be a responsible governing Republican majority if the American people give us a chance to do that," the party's top Senator Mitch McConnell said.
The veteran politician is locked in a tight race in Kentucky with resilient Democratic challenger Alison Lundergan Grimes, but two weekend polls showed McConnell extending his lead.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
"The wind is at our backs," Senator Rand Paul, a potential 2016 Republican presidential candidate, told CNN on Sunday.
"I think people are ready for new leadership."
Republicans have hammered home their message that a vote for Democrats is a vote for an unpopular Obama and his policies, in particular his still unpopular healthcare reform.
"This is a referendum on the president," Paul told NBC.
Republicans already hold the House of Representatives, where all 435 seats are in play, and forecasters predict they will gain seats.
Three top forecasters now give Republicans between a 70 per cent and 77 per cent chance of winning the Senate.
But however successful the Republicans are, a complete picture may not emerge on Tuesday.
There are strong prospects for runoffs in Louisiana and Georgia, where rules require a second round if winners do not earn more than 50 per cent of the vote.
Add to that a probable days-long ballot count in remote Alaska, where there is an unpredictable and tight race.
Louisiana's runoff is December 6, but a Georgia runoff would be on January 6, three days after the start of the new Congress.
