Long lines and malfunctioning machines have marred the first hours of voting in  midterm elections that will determine the balance of power in the US Congress and shape the future of Donald Trump’s presidency.

Polling places across the US struggled to meet the strong voter turnout for the polls, described in the US as a referendum on Mr Trump’s presidency.

All 435 seats in the US House of Representatives, 35 US Senate seats and 36 governorships are up for election.

Some of the biggest polling problems were reported in Georgia, a state with a hotly contested governor’s race, where some voters reported waiting up to three hours to vote.

At a polling place in Snellville, Georgia, the Associated Press reported more than 100 people took turns sitting in children’s chairs and on the floor as they waited in line for hours.

Voting machines at the Gwinnett County precinct did not work, so poll workers offered provisional paper ballots while trying to get a replacement machine.

One voter, Ontaria Woods, told the AP that about two dozen people who had come to vote left because of the lines.

Even before the polls opened, there were a wide variety of concerns across the US with voting and registration systems – from machines that changed voter selections, to registration forms thrown out because of clerical errors.

The first national elections since Mr Trump captured the White House in 2016 have been pitched as a vote on the polarising Republican’s presidency and his hardline policies, and a test of whether Democrats can turn the energy of the liberal anti-Trump resistance into victories at the ballot box.

“Everything we have achieved is at stake tomorrow,” Mr Trump told supporters in Fort Wayne, Indiana, at one of his three rallies to stoke turnout on the last day before the election.

The Democrats are favoured by election forecasters to pick up the minimum of 23 House seats they need for a majority, which would enable them to stymie Mr Trump’s legislative agenda and investigate his administration.

Republicans are expected to retain their slight majority in the US Senate, currently at two seats, which would let them retain the power to approve US Supreme Court and other judicial nominations on straight party-line votes.

But at least 64 House races remain competitive, according to a Reuters analysis of the three top nonpartisan forecasters, and Senate control was expected to come down to a half dozen close contests in Arizona, Nevada, Missouri, North Dakota, Indiana and Florida.

Democrats also threaten to recapture governor’s offices in battleground states such as Michigan, Wisconsin, Ohio and Pennsylvania, a potential help for the party in those states in the 2020 presidential race.

Voter turnout could be the highest for a midterm election in 50 years, experts predicted.

During a six-day blitz to wrap up the campaign, Mr Trump repeatedly raised fears about immigrants, issuing harsh warnings about a caravan of Central American migrants moving through Mexico toward the US border.

A debate about whether Mr Trump’s biting rhetoric encouraged extremists erupted in the campaign’s final weeks after pipe bombs were mailed to his top political rivals allegedly by a Trump supporter who was arrested and charged, and 11 people were fatally shot at a Pittsburgh synagogue.

But on the eve of the election, the President said in an interview he wished he had a softer tone during his first two years in office – even as he continued his relentless attacks on political rivals.

Mr Trump blamed the political vitriol on election season.

Former Democratic president Barack Obama hit the campaign trail in the election’s final days to challenge Mr Trump, questioning his policies and character.

“How we conduct ourselves in public life is on the ballot,” Mr Obama told election volunteers in suburban Virginia.

-with AAP