(CNN) With one day to go before the midterm elections , Americans face a choice that could shape the nation for years after a campaign that left it politically torn, at war with itself over race, and mourning tragedy.

Voters must decide on Tuesday whether to constrain President Donald Trump and his compliant Republicans after the first two years of a demagogic presidency that widened national divides and unfolded in a torrent of scandal. Trump also tested constitutional norms and engineered a sharp shift in the country's attitude toward the rest of the world.

Democrats continue to hold a double-digit lead over Republicans in a generic congressional ballot among likely voters, according to a new CNN Poll conducted by SSRS. The party's 55% to 42% advantage in the new poll mirrors their lead in early October and is about the same as the 10-point edge they held just after Labor Day.

But as they face their first chance to judge Trump's performance, they could also register satisfaction with a historically primed economy and a President who has kept many of his election promises, however controversial, and is running an undeniably consequential administration that has managed to engineer a generational conservative shift to the Supreme Court.

The first result would represent a rebuke to Trump's entire political approach: His failure to tame his volatile instincts in the interests of national unity and his unwillingness to embrace the presidency itself as a national trust.

The second would convey acquiescence for the President's scorched-earth tactics, indefatigable and domineering personality, fear-mongering warnings that the nation is under assault from an invading immigrant tide of dark-skinned criminals and approval of his creed of "America First" nationalism.

"You saw that barbed wire going up. That barbed wire -- yes sir, we have barbed wire going up. Because you know what? We're not letting these people invade our country," Trump said at a rally in Georgia on Sunday, defending his decision to dispatch troops to the border in what critics have branded a political "stunt."

While the campaign has seen intense skirmishes over health care, immigration, education and the best way to share the dividends of high growth, low unemployment and rising wages, Trump has, as he does all the time about everything else, made the campaign about himself.

In the most inflammatory closing argument of any campaign in modern memory, Trump seized on a group of migrants heading toward the southern US border from hundreds of miles away in Mexico as a metaphor for his hardline and racially insulting rhetoric on immigration. His searing nationalist rhetoric and tearing of cultural fault lines drew criticism that he had crossed a dangerous line after a gunman killed 11 people in a synagogue massacre in Pittsburgh and a Trump supporter mailed bombs to the President's top targets in politics and the media, including two former Presidents.

But it is a measure of the country's volatile political climate and the lessons of Trump's logic-defying win in 2016 that no one can say for sure how Tuesday night will unfold.

Trump v. Obama

At times on Sunday, it almost felt like Trump was running a campaign against the man whom he has defined himself against, his predecessor in the Oval Office, Barack Obama. The 44th president is making the most direct assault on Trump yet attempted by any prominent Democrat. Ten years to the day after he delivered his soaring victory speech in Grant Park, Chicago, Obama doubled down on hope, painting it as the antidote to what he said were the dark impulses exemplified by his successor, and warned America was at a crossroads.

"In the closing weeks of this election, we've seen repeated attempts to divide us with rhetoric, to try to turn us on one another," Obama said in Gary, Indiana, revisiting, a city familiar from his 2008 campaign.

"The good news is, Indiana, when you vote, you can reject that kind of politics," he said. "When you vote you can be a check on bad behavior. When you vote you can choose hope over fear."

Tuesday's election represents another clash between Trump's capacity to subvert political norms and the weight of history and electoral logic.

Omens look poor for Republicans, since Trump's approval rating sits between 40% and 45% in most polls and history suggests that first-term presidents who are that unpopular typically lead their parties to heavy losses.

Democrats are increasingly confident they can recapture the House of Representatives for the first time in eight years and are banking on a backlash against the President from voters who stayed home in 2016. Their path to power lies through more diverse, suburban and affluent districts where Trump's cultural warfare plays poorly.

But Trump's ironclad loyalty from a political base that sees him as a hero and a guardian of traditional, largely white, working-class life means that Republicans are strong favorites to keep the Senate, as vulnerable Democrats fight for political life in states where Trump won big two years ago like Indiana, Missouri, North Dakota and Montana.

Cultural and racial turmoil is also raging in several high-stakes gubernatorial races that, on a good night for Democrats, could produce the nation's first African American female governor, Stacey Abrams in Georgia, and Florida's first black chief executive in Andrew Gillum. In Wisconsin, an often liberal-leaning state that Trump crows about winning in 2016, a national political figure, Republican Gov. Scott Walker is facing yet another uphill battle to retain power.

What a shift in power would mean for the nation

The most likely outcome agreed upon by pollsters and pundits on Tuesday is one that would eloquently enshrine America's polarization -- Democrats would win a narrow majority in the House and Republicans would keep the Senate, and possibly gain a few seats.

Such a scenario would represent a significant threat to the White House, since it would empower Democratic committee chairmen to subject the White House to unprecedented scrutiny on everything from Trump's tax returns and business dealings to scandal in government agencies.

A Democratic-held House would also be a more receptive audience if special counsel Robert Mueller finds wrongdoing by the President and his campaign involving Russian election meddling, and theoretically would give Trump's congressional opponents the power to initiative impeachment proceedings.

While the loss of the House would cripple Trump's hopes of adding to a thin legislative agenda, retaining GOP control of the Senate would preserve the other key legacy thrust of the President's agenda, the remaking of the federal judiciary by Senate Majority Leader Mitch McConnell. Any Supreme Court vacancy over the next two years would allow Trump to consolidate his most significant achievement, the building of a conservative majority on the nation's top bench.

And if Republicans hold the Senate, any impeachment proceedings in the House are unlikely to result in a trial of the President and possible ejection from office, unless any charges that might come are especially egregious.

Should Democrats trigger a "blue wave" that sweeps away the Senate as well as the House, all bets are off for the Trump presidency. There would be questions asked about his fear-laden campaign strategy and how it could lead to an even bigger disaster in the 2020 presidential campaign that effectively begins on Wednesday morning.

If the GOP clings to the House and keeps the Senate, Trump would likely claim validation for his hardline leadership rooted in keeping his political base intact and vilifying opponents. That could enhance his power in a purge of restraining influences in his Cabinet that would augur a tumultuous period at home and abroad until November 2020.

The President's reputation for defying every political convention and omen would remain intact. And Democrats would have failed — yet again — to frame an effective counter-narrative to the President's strongman rule.