US midterms: all 435 House seats, tight Senate races and 39 governorships at stake

Voting is under way in the United States midterm elections. President Donald Trump has called the election the 'most important in our lifetime', Senator Bernie Sanders has called it the most important 'in modern American history', and former president Barack Obama has described it as a 'circuit-breaker' and the 'last, best chance' to rein in the president.

All eyes are on whether Americans will use the day as a referendum on Trump's second term, and whether Democrats will see a clean sweep of Congress.

Every one of the 435 seats in the House of Representatives is up for election, because members must defend their seats every two years.

Alongside that, 39 governorships are also up for election. Governors hold powerful positions leading their states.

In the Senate, attention will focus on seats currently held by Republicans in Texas, Kansas, Maine, Iowa, Ohio and Alaska. At the same time, there are close races to hold on to Democratic seats in Michigan and Georgia.

Major names such as James Talarico and Abdul El-Sayed are aiming for wins to flip Republican seats to the Democrats.

Both chambers of Congress, the House and the Senate, are in play. If either switches hands it will be a major obstacle for Trump and will sharply influence his agenda.

The House is needed to pass any legislation, and it can also open investigations or demand testimony from Trump officials. A Democratic Senate could also stymie nominations for positions such as the Supreme Court.

Many of those decisions come down to a simple majority vote. But even with Democrats in charge, steps such as impeachment remain far-fetched, as they need a simple majority in the House but a two-thirds majority in the Senate.

How long results take will depend on the state. Some states report results quickly, while others are notoriously slow. Many states are allowed to process absentee and mail-in ballots before election day, so they can report a tranche of results soon after polls close, but some cannot.

Democrats tend to vote earlier in greater numbers and Republicans tend to vote more in person, so early results can present a kind of blue mirage that comes into sharper focus as counting continues.

Margin will be decisive in the end. If Democrats win a comfortable number of seats in the House, the Senate or both, a result could come early. But if key races are tight and the final breakdown of Congress comes down to a seat or two, counting could stretch into a long week.

Midterms usually see far fewer people vote than presidential elections; turnout is generally between 40 and 50 percent of eligible voters. Early voting this year, however, is significantly higher than during the last midterm election.

Election rules also differ in every state. Eight states mail ballots to all eligible voters, some allow mail-in ballots with no reason needed, while others require an excuse. Voter ID laws also vary, with some states requiring a photo ID, some an ID with just a name and others no ID at all. Polls generally close between 7pm and 9pm local time, and anyone still in line at closing time can cast a ballot.