Democrats brace for defeat in US elections

Americans are set to vote in legislative elections expected to inflict a stinging defeat on President Barack Obama, with Republicans poised to wrest control of the Senate from his Democrats.

The party of an incumbent US president historically fares badly in elections in the middle of his second term, and this time is expected to be no different for Obama.

Many Republicans have essentially based their campaigns on attacks against the president and his policies, particularly his healthcare plan. And many Democrats even avoided including the president in their campaign rallies and photo ops.

Voting in one of the most pivotal midterm elections in years will start at 6.00am (2200 AEDT) on Tuesday with polling stations opening in the eastern states.

Although the economy has been improving gradually since the recession of 2008, the national mood is far from buoyant.

Much attention has focused on recent flashpoint crises such as Ebola and advances by the Islamic State jihadists in Syria and Iraq.

Democrats currently hold a 55-45 seat advantage in the US Senate, while Republicans control the House of Representatives.

If Republicans take a net six seats in the Senate, Obama will spend his last two years in office facing a hostile congress as he contends with Ebola, Islamic extremists and improving the economy.

Recent polls show Republicans pulling ahead in the battle for power in Washington despite races in Alaska, North Carolina and other states remaining very close, and they expressed confidence in the home stretch.

"We intend to be a responsible governing Republican majority," the party's top Senator Mitch McConnell told ABC News as he barnstormed his state of Kentucky.

The veteran politician is locked in the tightest race of his career with resilient Democratic challenger Alison Lundergan Grimes, but two weekend polls showed McConnell extending his lead.

"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 a tarnished Obama and his policies.

"This is a referendum on the president," Paul told NBC.

In the House of Representatives, where all 435 seats are in play, experts predict the Republicans will gain more seats. A third of the 100-seat Senate is up for grabs.

Three top forecasters now give Republicans between a 70 per cent and 77 per cent chance of winning the Senate as well.

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, which would also delay matters.

Louisiana's runoff is December 6, but a Georgia runoff would be on January 6, three days after the start of the new congress.