Results at 6.30pm NZT*
House: Democrats 210 Republicans 189 | 218 needed - Democrats gain control
Senate: Democrats 42 Republicans 51 | 51 needed - Republicans hold control
State of play: Democrats to take the House, Republicans hold the Senate. That leaves Congress split and reduces the ability of President Trump to get the law changes he wants.
6.22pm: That wraps up these updates with bulk of the races now called, appearing to confirm the Democrats' House win and the Republicans' hold of the Senate. Getting anything done in Washington now is going to be interesting.
6.03pm: 1 NEWS US Correspondent Rebecca Wright, reporting from a Republican gathering in Phoenix, Arizona, says there was a "huge roar of relief" there when the Senate was retained. She says there has been record turnout in many parts of the country.
"Washington DC is heading for gridlock," she says.
5.47pm: Democrat Nancy Pelosi, currently the minority leader in the House, says "tomorrow will be a new day in America" as her party closes in on control of that chamber. President Trump has apparently called to congratulate her.
5.35pm: With the Democrats taking over control of the House, commentators say a number of investigations into the president are likely. ABC News reports that Democrats on the House Oversight Committee will target the Muslim travel ban, the response to Hurricane Maria, the Affordable Health Care Act in court, and administration use of chartered flights.
5.28pm: What did we say (see 5.10pm entry below), Trump breaks silence to call the results a "tremendous success".
5.10pm: President Trump will not speak this evening, the White House says. It is now 11pm in Washington, though he has been known to send tweets at any time of day or night, so it's worth keeping an eye on @realDonaldTrump for his reaction to today's results.
4.55pm: ABC calls the House for the Democrats, saying they have the 23 gains they need. It has already called the Senate for the Republicans.
4.51pm: Another Senate blow for the Democrats - incumbent Heidi Heitkamp loses to Republican Kevin Cramer in North Dakota.
4.45pm: Firsts: The House is getting its first two Muslim women and Massachusetts is getting its first black congresswoman while Arizona and Tennessee stand to elect their first woman senators.
4.40pm: Democrats continue to gain ground in their fight for control of the House, picking up key seats in Florida, Pennsylvania and Minnesota. But in the Senate they suffered setbacks in Indiana, Tennessee and Texas.
4.29pm: Republican Ted Cruz has defeated Democrat Beto O'Rourke in Texas. O'Rourke is seen as a possible future Democrat presidential candidate.
4.18pm: ABC News calls the Senate for the Republicans.
4.13pm: The Democrat gain in the House, according to ABC News, is now 11 seats.
3.54pm: Fox News calls the House for the Democrats. If correct, that's a blow to Trump as it will make it harder for him to get the legislation he wants and will likely leave Congress split between a Democrat-controlled House and a Republican-controlled Senate.
Other networks have not yet called the House for the Democrats but ABC has 10 gains for them at this point. And Five Thirty-Eight now has them back up at 79% chance to win the House.
3.40pm: The White House is calling it not a "blue wave" for the Democrats, but maybe a blue "ripple".
3.38pm: Democratic incumbents who were re-elected today include New Jersey Senator Bob Menendez, Sherrod Brown of Ohio and Bob Casey of Pennsylvania. The victories by Brown and Casey underscored that Trump's 2016 dominance of the Midwest, once a Democratic stronghold, was not a lock.
3.25pm: Some significant results from the day so far:
Republican Andy Barr of Kentucky won a close-fought race that Democrats had targeted in a bid to shift the House to Democratic control.
In Indiana, Greg Pence, an older brother of Vice-President Mike Pence, has won a heavily Republican House seat that his famous sibling once held.
Republican Barbara Comstock of Virginia was the first congressional incumbent to lose. Democratic Senator Jennifer Wexton won an easy victory over her.
Donna Shalala has won a House seat in Florida, making her the first Democrat to flip a Republican seat.
3.09pm: Trump-supporting Fox News' "probability meter" gives a 66% chance of the Republicans controlling the Senate, and 79% of the Democrats winning the House.
3.02pm: Of seven potential gains for the Democrats in Florida, the Republicans have held on to five.
2.57pm: Away from the main national action, Kim Davis - the county clerk in Kentucky who refused to sign gay couples’ marriage licences - has lost her re-election bid.
2.50pm: Democrat incumbent in Indiana, Joe Donnelly, projected to lose - bad news for the Democrats, further reducing their chance of wining the Senate.
2.32pm: The results are mixed for the Democrats so far, with only one gain and few breakthroughs in Republican areas of Florida for example. Five Thirty-Eight's forecasts now have them at just 3% likelihood to gain the Senate and 54% to gain the House.
2.24pm: Elizabeth Warren, Massachusetts Democrat Senator, holds her seat. She's a potential Democrat presidential candidate in 2020.
2.02pm: A flurry of results come in at the point when half the polls in the country are now closed, giving a number of seats in both the House and the Senate to the Democrats.
1.53pm: ABC projects a House gain for the Democrats in Virginia for Jennifer Wexford.
1.32pm: Pollsters Five Thirty-Eight are currently predicting a 13% chance of the Democrats controlling the Senate, but an 87% chance of them controlling the House.
1.27pm: Nearly 40 percent of voters cast their ballots to express opposition to the president, according to AP VoteCast, a national survey of the electorate, while about 25 percent said they voted to express support for Trump
1.15pm: Bernie Sanders, Independent Senator from Vermont - who challenged Hillary Clinton to be the Democrat candidate at the last presidential election - is projected to hold his seat.
1.03pm: The US networks are now calling some of the seats, with some early wins for the Republicans.
12.46pm: Polls close in six eastern states in 14 minutes, at 1pm NZ time. Exit polls will come after that. CNN is already calling two House seats for the Republicans.
11.42am: A poll of 113,000 voters nationwide shows the key issues most are concerned about are: healthcare, immigration and Donald Trump.
11.14am: Going into election day, the Democrats needed to gain two Senate seats to win a majority, assuming all their incumbents were re-elected, an unlikely outcome. But their target list was limited: They had a plausible chance of winning Republican-held seats only in Arizona, Nevada, Tennessee and Texas.
11.08am: The midterm elections brought a surge of female candidates to ballots across the country. In the House, 237 women were on the ballot as major-party candidates. Women hold 84 out of 435 House seats, a record number. If more than 84 of those women candidates won on Tuesday, it would set a record for the number of women serving in the House.
10.24am: Long lines and malfunctioning machines marred the first hours of voting in some precincts, including in Georgia, where some voters reported waiting up to three hours to vote. More than 39 million Americans have already voted, either by mail or in person, breaking early voting records across 37 states.
- with Associated Press
- Election results via ABC News