Americans are voting Tuesday in an election that analysts are predicting will give Republicans control of Congress for the first time in a decade, likely generating new political disputes with President Barack Obama during the Democrat's last two years in office.
Republicans, generally regarded as the more conservative political party in the United States, already control the 435-member House of Representatives and could add several seats to the 233 they already hold.
The Republicans could also take command of the 100-member Senate, where the more liberal Democrats now hold a 55-seat working majority.
All House seats and a third of the Senate seats are at stake in the election. Obama, who won a second four-year term in 2012, is not on the ballot, although he has said that his policies are.
But with the president's approval rating mired in the low 40 percent range, Republican candidates could pick up several Senate seats in states Obama lost two years ago, even as he won handily nationwide.
The latest opinion surveys show Republican candidates will easily win Senate races in West Virginia, South Dakota and Montana, which are currently held by Democratic lawmakers.
Polling also shows Republicans poised to win Senate races in Iowa, Colorado and Alaska, although it may be several hours - or even days - before the final results are known. And two races in the Deep South, Georgia and Louisiana, are so close they could certainly head into runoff elections.
Senate Republican Leader Mitch McConnell, himself embroiled in a competitive re-election race in Kentucky, expressed confidence about his party's chances during a final campaign appearance Monday in Louisville.
"Victory is in the air and we're going to bring it home tomorrow night," McConnell said.
In a widespread campaign tactic, the Republican Senate candidates in these states have sought to link Democratic incumbents to Obama's unpopularity.
Numerous Democratic officeholders declined to invite Obama to campaign for them and often cited instances where they disagreed with his performance, such as on health care or energy issues.
The accuracy of pre-election U.S. political surveys has often been erratic, with some polling turning out to be way off the mark once the elections were actually held.
Even as several Senate races are deemed too close to call, analysts said Republicans have about a 70 percent chance of picking up at least six seats to control the Senate.
If the Republicans do control Congress, it could presage new disputes with the president over his signature legislative achievement, massive national health care reforms that have allowed millions of people to secure insurance coverage they could not previously afford.
Many Republicans view it as excessive government involvement in peoples' health care and called for repeal of the law.
Many Republicans have also attacked Obama's handling of the current Ebola crisis, called for approval of an oil pipeline from Canada through the central U.S., and sought a curb on government regulation of businesses.
Some opposition lawmakers have also disputed his handling of Russia's intervention in Ukraine and U.S. airstrikes against Islamic State militants in Iraq and Syria.
In the United States, the two political parties are also feuding over spending and tax policies and immigration reforms.
Obama has vowed to set new immigration rules by executive order by the end of the year, after the House balked at acting on comprehensive reforms approved by the Senate.
Some Republicans already are saying they will seek to block the president from unilaterally changing the country's immigration policies to allow millions of migrants who entered illegally to stay in the United States.