Americans are voting in mid-term elections which will decide who controls the Senate and pave the way for the 2016 race for the White House.
Polling booths opened in the eastern states at 06:00 local time (11:00 GMT).
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 Mr 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.