Millions vote in US mid-term elections

Record campaign spending may result in fewer votes per dollar than ever before if early fears of lacklustre turnout in America’s midterm elections on Tuesday prove an accurate reflection of the cautious races fought in many states.

Polling booths opened in the eastern states at 06:00 local time and the Republicans, who already control the House of Representatives, need to gain just six seats to take the Senate.

Meanwhile the Democrats are battling to stay ahead as President Barack Obama's approval ratings fall to the lowest they have been since he was elected.

Many analysts predict a Republican victory as Obama's popularity rate has failed to climb much above 40%, despite recent improvements in the economy.

"This is a referendum on the president," Republican senator and potential 2016 presidential candidate Rand Paul told NBC's Meet the Press at the weekend.

Democrats in particular are worried that low turnout may cost them control of the Senate if they fail to motivate their supporters to vote in seats where many candidates have run predominantly negative campaigns aimed at highlighting extreme Republican policies.

Republicans have focused instead on attacking the track record of Barack Obama, forcing the president to keep a low profile, and were careful to pick less outspoken candidates than in previous years for fear of alienating moderate voters.

The combination has led to a low-key campaign on both sides and although there are few consistent ways of tracking turnout in advance of polls closing, there was little evidence on Tuesday morning of a last-minute rush to the polls.