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.
The Daily Beast has examined that "unthinkable" scenario in some depth. "I wouldn't say it's as important as '16, but it's right up there". Trump's enemies are gambling everything on winning.
Healthcare and immigration were the top issues on voters' minds as they cast their ballots, according to an exit poll survey conducted by the Associated Press, and 64% of those surveyed said Trump was a factor in their voting choice.
While Trump is not on the ballot, the most polarizing U.S. president for decades has put himself at the center of every issue.
Early voting leading up to Tuesday's midterm election revealed a wide variety of concerns with voting and registration systems around the country - from machines that changed voter selections to registration forms tossed out because of clerical errors. In the midterms, because there is no presidential election, there is no electoral college.
He claimed Monday that Democrats wanted to make America "a giant sanctuary city for drug dealers, predators, and bloodthirsty MS-13 killers". That playing field, more and more people - average, ordinary Americans - see this unfairness, and it's just basic unfairness.
The party hopes younger voters, suburban moderates and minorities will be drawn to the polls to react against the president's rhetoric.
To go in person, you need to find your closest polling station and required ID, possible via the US Foundation website.
Mid-term elections are often seen as a referendum on the president and there is no shortage of opinions this time around about the current occupant of the White House. Wins for Democrats in key swing states like Florida, Ohio and Wisconsin would not only lead to far more progressive state governments but also give the party influence in the redistricting process scheduled to take place after 2020. What if all the pollsters and pundits have missed a surge of silent Trump supporters?
Mighty Winx wins record fourth Cox Plate
Chapman caused a stir at the barrier draw for the Cox Plate when he questioned whether Winx was entitled to be rated the world's best racehorse.
Regardless of the numerical outcome, the fault lines in the United States look set to become even more apparent - divided between a liberal-moderate electorate concentrated in urban and coastal areas who mostly despise the President and a conservative-religious-blue collar voters spread across a vast swathe in the middle of the country who broadly love him.
Democrats are also expected to pick up an open seat that Clinton won in Washington state that has been held by the GOP. Ironically, President Trump's campaign ended in a rally where his supporters broke into the gospel hymn Amazing Grace when someone in the audience had a medical emergency. Democratic Rep. Beto O'Rourke, a darling of progressives from coast to coast, raised record contributions but faced long odds of ousting tea party Sen.
Democrats are bringing in their own heavy hitter while campaigning to boost Obamacare and bring President Trump's legislative accomplishments to a standstill. The Tea Party movement grew as a response to that, propelling Republicans to a crushing victory in the midterms that year.
Democrats are acutely aware of the risks they are facing this time around.
In an interview with Fox and Friends, Judge Andrew Napolitano have his assessment of what a Democratic majority in the House would mean for the Trump administration.
"In one corner would be the class warfare Sanders wing, and in another would be the intersectional, social issues left, and in a third would be the establishment Liberals", Baer said.
In Pittsburgh, where residents just finished burying those gunned down at the Tree of Life synagogue, some voters saw their Election Day decisions as a way to send a message that the country is headed down a dark and unsafe path.
Obama has been their most effective voice in this midterm campaign, but he has already served two terms as president.
"Obama's got a unique combination of things, and that is going to be quite hard to replicate", Professor Brendon O'Connor told news.com.au.