The Republican Party has taken control of the Senate by picking up at least six seats from the Democrats
They have also strengthened their grip on the House of Representatives
Ballot initiatives expanding marijuana use and increasing minimum wage passed in some states
The economy, government dysfunction and President Obama's unpopularity were key issues
Head precinct judge Deloris Reid-Smith (left) reads the voters' oath to poll workers before opening the polls at the Grove Presbyterian Church in Charlotte, North Carolina.
Image source, ReutersThe US states listed as having the most powerful voters also have very small populations, including Wyoming, Vermont, Alaska, North Dakota and Rhode Island, writes, external Jaime Fuller for the Washington Post.
tweets, external: It's show time folks -- this is what the dudes in the three point hats went to war for so you could go to the voting booth -- #Vote
writes:, external The outcome is likely to be a more conservative America and, if the Senate does turn Republican, a country all the more inclined to block Mr Obama's progressive agenda and, where possible, roll it back. However, Republicans know their new majority might last only two years, assuming, that is, they first obtain it by picking up those six net seats, as many predict they will.
The BBC will have live TV coverage of the US mid-term election results from midnight GMT. Rajesh Mirchandani will be using a virtual reality studio to take us through the election night results.
Jane O'Brien
BBC News, Arlington, Virginia
You would assume that if the Republicans take control of Congress, President Obama would find it very difficult to get any of his legislation through. He's already had six tough years with Republicans blocking most of his initiatives.
But there is also a theory that if the Republicans do gain control of both chambers then they will have two years to prove they can govern, and that's important as we head into the 2016 presidential elections. So it may be you'll find an atmosphere of slightly more reconciliation and working together. It's certainly something Americans will be hoping for.
A Democrat supporter mans an information table in Virginia. Voters there are choosing between Democrat Mark Warner and Republican Ed Gillespie for the Senate.
Image source, EPACan the Republicans pick up the "magic six" seats needed to gain control of the Senate? Here is the BBC's guide to six US Senate races to watch.
tweets, external: When you #vote today, think about which candidate is most serious about increasing educational opportunity & improving student achievement.
This is the last time voters will be able to weigh in on the Obama presidency, with his remaining two years in office likely to be shaped by the mid-term results. Mr Obama has said on several occasions he is being "challenged" in these midterms and that in every state his "issues are on the ballot," a statement which is thought to have hurt vulnerable Democrats in tight races.
@JacRas11 tweets:, external You know you work in politics when your up at 5:30 a.m on Election Day.... #Midterms2014 #Work #DDay
If the Republicans win the Senate, the day-to-day running of the chamber would fall to Mitch McConnell, currently the minority leader. The president would retain his veto on bills but Mr McConnell could set the Senate agenda. Our blog explains what difference a Republican Senate would make.
Image source, AP@alcestian tweets:, external I had to wake up extra early so I could vote before my 12.5 hour shift today, so the rest of you- get up & make time to vote! #Midterms2014
Jon Sopel
North America editor
I have spoken to Democrats and Republicans and they all said the same thing - they were sick of the partisan posturing, the gridlock, the inability to work together, the dysfunctional relationship between Congress and White House, between legislature and executive.
All of which begs the question - what difference will it make if the Republicans do take control of the Senate?
Some residents in Virginia are already up and queuing to vote.
Image source, EPA@Britt_Thomason tweets:, external I hope there is no runoff... Don't think I can take 2 more months of these commercials... #Georgia #Midterms2014
@chucktodd
tweets, external: It's E Day! The most American of days. Never take this for granted.
The first polling stations to open on Tuesday were in the eastern states of Kentucky, Maine, New Hampshire, New Jersey, New York and Virginia, AFP news agency reports.
Mid-terms also typically favour the party that is not in power - which is bad news for US President Barack Obama, whose approval ratings fall to the lowest they have been since he was elected.
Although control of the Senate is at stake, turnout is still expected to be low in the mid-terms. Voters tend to care more about presidential elections than seats in the House and the Senate.
