A turbulent election season that tested President Donald Trump’s slash-and-burn political style against the strength of the Democratic resistance comes to a close as Americans cast ballots in the first national election of the Trump era on Tuesday.

US voters will decide whether Trump will keep his Republican majority in Congress or face a hostile Democratic majority after a bitter campaign for midterm elections described by both sides as a battle for America's soul.

For almost two years, Trump's rule-breaking, sometimes chaotic administration has enjoyed a largely free hand from the twin Republican-controlled chambers, but the midterms could finally see his wings clipped.

The entire 435-member House of Representatives and a third of the 100-seat Senate are up for grabs.

Anxious Republicans privately expressed confidence in their narrow Senate majority but feared the House was slipping away. Trump, the GOP’s chief messenger, warned that significant Democratic victories would trigger devastating consequences.

“If the radical Democrats take power they will take a wrecking ball to our economy and our future,” Trump declared in Cleveland, using the same heated rhetoric that has defined much of his presidency. He added: “The Democrat agenda is a socialist nightmare.”

Tuesday’s results will be colored by the dramatically different landscapes in the fight for the House and Senate.

Most top House races are set in America’s suburbs where more educated and affluent voters in both parties have soured on Trump’s presidency, despite the strength of the national economy. Democrats were buoyed by a wave of Republican retirements and an overwhelming fundraising advantage.

They need to pick up two dozen seats to claim the House majority.

Democrats face a far more difficult challenge in the Senate, where they are almost exclusively on defense in rural states where Trump remains popular. Democratic Senate incumbents are up for re-election, for example, in North Dakota, West Virginia, and Montana — states Trump carried by 30 percentage points on average two years ago.

Democrats need to win two seats to claim the Senate majority.

Given Trump’s stunning victory in 2016, few were confident in their predictions.

“I feel less comfortable making a prediction today than I have in two decades,” Republican pollster Frank Luntz said.

'Awakening of the Democratic Party'

Democrats, whose very relevance in the Trump era depended on winning at least one chamber of Congress, were laser-focused on health care as they predicted victories that would break up the GOP’s monopoly in Washington and state governments.

“They’ve had two years to find out what it’s like to have an unhinged person in the White House,” said Washington Gov. Jay Inslee, who leads the Democratic Governors Association. “It’s an awakening of the Democratic Party.”

Democrats could derail Trump’s legislative agenda for the next two years should they win control of the House or the Senate. Perhaps more important, they would claim subpoena power to investigate Trump’s personal and professional shortcomings.

Some Democrats have already vowed to force the release of his tax returns. Others have pledged to pursue impeachment, although removal from office is unlikely so long as the GOP controls the Senate or even maintains a healthy minority.

Democrats’ fate depends upon a delicate coalition of infrequent voters — particularly young people and minorities — who traditionally shun midterm elections.

If ever there was an off-year election for younger voters to break tradition, this is it. Young voters promised to vote in record numbers as they waged mass protests in the wake of the February mass shooting at a Parkland, Florida, high school that left 17 students and staff dead.

Democrats are drawing strength from women and college-educated voters in general, who swung decidedly against Trump since his election. Polling suggests the Republican coalition is increasingly older, whiter, more male and less likely to have a college degree.

Diversity in national elections

Democrats boast record diversity on the ballot.

Three states could elect their first African-American governors, while several others are running LGBT candidates and Muslims. A record number of women are also running for Senate, House, governorships and state legislative seats.

“The vast majority of women voters are angry, frustrated and they are really done with seeing where the Republican Party is taking them, particularly as it related to heath care and civility,” said Stephanie Schriock, who leads EMILY’s List, a group that help elect Democratic women. “You’re going to see the largest gender gap we’ve ever seen.”

The political realignment, defined by race, gender and education, could re-shape US politics for a generation. The demographic shifts also reflect each party’s closing argument.

While the economy continues to thrive, Trump has spent much of the campaign’s final days railing against a caravan of Latin American immigrants seeking asylum at the US border. He dispatched more than 5,000 troops to the region, suggesting soldiers would use lethal force against migrants who throw rocks, before later reversing himself.

Republicans have privately encouraged the president to back off, to no avail.

Democrats, meanwhile, have beat their drum on health care.

“Health care is on the ballot,” former President Barack Obama told Democratic volunteers in Virginia. “Health care for millions of people. You vote, you might save a life.”

Bernie Sanders, the leftist populist who some feel would have had a better chance than Clinton to take on Trump in 2016, lashed out Monday at the president, calling him a "pathological liar".

"He is a sexist, a racist, a homophobe, a xenophobe and a religious bigot. He is trying to do what we have never seen in the modern history of this country, to do what he is doing right now, to gain votes by trying to divide the American people up based on where we came from," Sanders said on SiriusXM Progress radio.