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
The famous dome on the US Capitol building in Washington is currently covered in scaffolding. But throughout this campaign, most voters have been saying it will take more than a facelift to "fix" Congress.
Image source, AFPBut we shouldn't forget there's a battle for the lower chamber too, the House of Representatives, where the Republicans have a lead of 233 to 199. They are expected to maintain or increase that, tightening their grip on the House.
Jon Sopel
North America editor
tweets, external: Polling and stats genius @NateSilver538 latest blog says 76% chance Republicans will take Senate
Expectations are that turnout in 2014 will not be as high as in previous mid-term years - 2006 and 2010 - when the Iraq War and the president's healthcare overhaul were galvanising issues.
Image source, AFPSo with a dizzying number of contests happening tonight, we've whittled them all down to six Senate races that really matter: Six US Senate races to watch
Republicans haven't controlled the US Senate since 2006. If they win the big prize, the man in charge of the Senate will be current Minority Leader Mitch McConnell, looking happy here while voting in Louisville, Kentucky.
Image source, European photo agencyThe grand prize is the US Senate, where Republicans are vying for control and need six seats to wrestle it from the Democrats. Republicans already control the US House of Representatives and are unlikely to lose it.
For those just joining us, it's time for a recap of what's at stake tonight - 36 out of 50 state governors, 36 of 100 seats in the Senate and all 435 districts in the House of Representatives, plus countless state and local offices.
Nick Bryant
BBC News, Kentucky
tweets, external: "Team Mitch" paraphernalia ready to go for what's billed as the "victory party" in Louisville @BBCNewsUS
tweets, external: Polls are still open. Do not put voting off. http://gop.cm/6011STxS The next generation is looking to you.
tweets, external: Closest governors' races: Democrats favored to win in ME, CO, FL, CT, IL, RI, and KS. Republicans in MI and WI: http://53eig.ht/10hyJgd
Facebook has developed an interactive dashboard, external to show which users have voted and where. The map is updating in real time as users share what they've voted on Facebook.
US media say Tim Scott is tipped to be the first African-American elected as a senator from the South. Mr Scott is already a senator, but he was appointed after his predecessor Jim DeMint retired.
Image source, Getty ImagesWant to know who is behind all the campaign adverts from groups with names like "Patriot Majority", "Women Vote!" and "Crossroads GPS"? Well, you can't. Groups like these are funded by "dark money", which can be raised without any requirement to identify the donors. The BBC's Anthony Zurcher looks at what this means for the US political process.
Bahman Kalbasi
BBC Persian, Kentucky
tweets, external: #Kentucky: Reporting from a voting stn in #Louisville. Sherif told me the turn out is V high for midterms. @BBCNewsUS
Tara McKelvey
BBC News, Kansas
Polls have shown the Kansas Senate race between Republican Senator Pat Roberts, and Greg Orman, an independent, is a dead heat.
Both have their supporters in Topeka, a town filled with boarded-up businesses like this gas station. Yet many people have told me they're voting on social issues, not economic ones. That usually means a vote for Roberts.
A billboard above the abandoned gas station says: "Homosexuality is sin". It was paid for by Westboro Baptist, a conservative church.
Nick Bryant
BBC News, Kentucky
Seniority is clearly a highly valued commodity in Kentucky, where 72-year-old Mitch McConnell is seeking a sixth term. He has already broken the record as the longest serving Senator in the state's history.
Seniority usually brings with it pork (federal money), and that's appetising for many of the voters we've been speaking to.
Youth seems to be his opponent's major handicap. Alison Lundergan Grimes is just 35 years old. Why have a junior senator, many people clearly think, when Kentucky could be represented by the most powerful man in the Senate?
Image source, Reuters
Kim Ghattas
BBC News, Arlington, Virginia
tweets, external: Fun slow day covering mid term elex for @bbcworldservice @BBCNewsUS with @trowynt in Virginia. Trickle of voters.
Jane O'Brien
BBC News, Arlington, Virginia
tweets, external: . @carrolldoherty tells me only 43 percent of Americans know who controls Congress now. Will they care who takes the Senate tonight?
tweets:, external Among conservatives, more say Christie/Rand/Jeb would NOT make good prezes than say they would http://wapo.st/1ofO7Fv , external