The day of reckoning for American politics has nearly arrived.

Voters on Tuesday will decide the $US5 billion ($A6.95 billion) debate between President Donald Trump's take-no-prisoner politics and the Democratic Party's super-charged campaign to end the GOP's monopoly in Washington and statehouses across the nation.

There are indications that an oft-discussed "blue wave" may help Democrats seize control of at least one chamber of Congress.

But two years after an election that proved polls and prognosticators wrong, nothing is certain on the eve of the first nationwide elections of the Trump presidency.

"I don't think there's a Democrat in this country that doesn't have a little angst left over from 2016 deep down," said Stephanie Schriock, president of EMILY's List, which spent more than ever before - nearly $US60 million in all - to support Democratic women this campaign season.

"Everything matters and everything's at stake," Schriock said.

All 435 seats in the US House are up for re-election. And 35 Senate seats are in play, as are almost 40 governorships and the balance of power in virtually every state legislature.

While he is not on the ballot, Trump himself has acknowledged that the 2018 mid-terms, above all, represent a referendum on his presidency.

Should Democrats win control of the House, as strategists in both parties suggest is likely, they could derail Trump's legislative agenda for the next two years. Perhaps more importantly, they would also win subpoena power to investigate the president's many personal and professional missteps.

Tuesday's elections will also test the strength of a Trump-era political realignment defined by evolving divisions among voters by race, gender and especially education.

Trump's Republican coalition is increasingly becoming older, whiter, more male and less likely to have a college degree. Democrats are relying more upon women, ethnic minorities, young people and college graduates.

Minorities and women have increasingly fled Trump's Republican Party, turned off by his chaotic leadership style and xenophobic rhetoric. Blue-collar men, however, have embraced the unconventional president.

A nationwide poll released on Sunday by NBC News and The Wall Street Journal details the depth of the demographic shifts.

Democrats led with likely African-American voters (84 per cent to 8 per cent), Latinos (57 per cent to 29 per cent), voters between the ages of 18-34 (57 per cent to 34 per cent), women (55 per cent to 37 per cent) and independents (35 per cent to 23 per cent).

Among white college-educated women, Democrats enjoy a 28-point advantage: 61 per cent to 33 per cent.

On the other side, Republicans led with voters between the ages of 50 and 64 (52 per cent to 43 per cent), men (50 per cent to 43 per cent) and whites (50 per cent to 44 per cent). And among white men without college degrees, Republicans led 65 per cent to 30 per cent.

Former President Barack Obama seized on the differences between the parties in a final-days scramble to motivate voters across the nation.

"One election won't eliminate racism, sexism or homophobia," Obama said during an appearance in Florida. "It's not going to happen in one election. But it'll be a start."

Trump has delivered a very different closing argument, railing against Latin American immigrants seeking asylum at the US border.

With the walking caravan weeks away, Trump has dispatched troops and said soldiers would use lethal force against migrants who throw rocks, before later reversing himself.

The hyper-charged environment is expected to drive record turnout in some places, but on the eve of the election, it's far from certain which side will show up in the greatest numbers.

Democrats are most optimistic about the House, a sprawling battlefield extending from Alaska to Florida. Most top races, however, are set in America's suburbs where more educated and affluent voters in both parties have soured on Trump's turbulent presidency, despite the strength of the national economy.

Democrats 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 defence in rural states where Trump remains popular.

Democrats need to win two seats to claim the Senate majority, although most political operatives in both parties expect Republicans to add to their majority.

While Trump is prepared to claim victory if his party retains Senate control, at least one prominent ally fears that losing even one chamber of Congress could be disastrous.

"If they take back the House, he essentially will become a lame-duck president, and he won't win re-election," said Amy Kremer, a tea party activist who leads the group Women for Trump.

"They'll do anything and everything they can to impeach him," she said.

By Election Day, both sides are expected to have spent more than $US5 billion, according to the Centre for Responsive Politics. The flood of campaign cash, a midterm record, has been overwhelmingly fuelled by energy on the left.