Until all the votes are counted, no one can be sure where the chips will fall, but there is increasing evidence that President Donald Trump's Republican Party could lose control of the House of Representatives.
Mr Obama accused Mr Trump of lying and "fear-mongering", warning Democrats not to get distracted.
A new Washington Post-ABC News poll on Sunday suggested that while Democrats retain an edge in their battle for the House, Republicans could profit from increasingly positive assessments of the economy and by Trump's harsh focus on border security.
"They want to impose socialism on our country".
The poll also finds 8 in 10 Americans think the country is more deeply divided this year on major issues than it has been in the past several years.
Opinion polls and election forecasters have made Democrats favourites on Tuesday to pick up the 23 seats they need to capture a majority in the U.S. House of Representatives, which would enable them to stymie Trump's legislative agenda and investigate his administration. "And [voters are] going to say Republicans have offered a record of results and Democrats are offering resistance and going back". Subscribe to the Standard Group SMS service by texting "NEWS" to 22840.
Republicans now control 29 governorships to 21 for the Democrats.
Monday will be a barnstormer for Trump, who will make stops in OH and in before a final campaign pitch in Missouri, where he is trying to knock Democratic Senator Claire McCaskill out of office.
Trump is set to campaign Sunday in Macon, Georgia, for Republican gubernatorial candidate Brian Kemp and in Chattanooga, Tennessee, on behalf of Republican Senate candidate Marsha Blackburn.
"The contrast in this election could not be more clear".
With opinion polls showing dozens of tight USA congressional and gubernatorial races in Tuesday's election, the current and former presidents said the results would determine what kind of country Americans live in for the next two years.
"They've racked up enough indictments to fill a football team", Obama scoffed.More news: Football leaks exposes Premier League clubs at heart of planned European Superleague
And while Obama said Democrats would support a "kinder version of America", he did what many Democratic candidates have refrained from doing: directly challenging the president.
"There may be centrist legislators left but they dare not admit it", says Hart, who believes Democrats are raring for a fight.
"The level of fervor, the level of fever is very strong on the Republican side", he said. "There is a lot of energy out there". But a healthy economy tends to favour the incumbent - and the U.S. economy has been growing with rare vigour.
Anti-Trump Canadians may be breathing a sigh of relief at the prospect, but some Canadian policy experts say a Democratic win is not necessarily better for the Canadian economy or Canadian business.
White women without college degrees prefer Republicans by a 12-point margin, though white women with degrees clearly prefer Democrats, 61 percent to 33 percent. The gap was 9 points in the previous edition of the poll.
Another wild card: The campaign's closing days come just a week after a gunman, who allegedly hated immigrants and Jews, killed 11 people at a Pittsburgh synagogue.
Obama also condemned Trump's escalating racism going into the midterms.
Both events prompted Mr Trump's critics to blame the president for helping to create an atmosphere in which the attackers felt able to commit their crimes.
But one of USA voters' major concerns is healthcare, as Republicans have been trying, so far unsuccessfully, to kill off Mr Obama's Affordable Care Act.
"We're not letting these people invade our country", Trump declared.
Democrats meantime insist that only they will protect the healthcare gains made under President Barack Obama, that Trump has employed inhumane measures to keep migrants out, and that the divisiveness he has fostered must end.
"There's got to be consequences when people don't tell the truth, when words stop meaning anything".