All 435 seats in the US House of Representatives, 35 US Senate seats and 36 governorships are up for grabs in elections focused on dozens of competitive races from coast to coast that opinion polls show could go either way.
The comments came as Trump prepared for a final, three-state rally blitz as he tries to keep Congress in Republican control and stave off losses that could profoundly change his presidency.
The president has vowed to continue pushing for significant funding for a border wall, which Republicans may feel more comfortable with backing if voters hand them power again.
"After 11 rallies in 8 states and weeks of campaigning for Republican candidates, the President will spend today making phone calls, monitoring Congressional, Senate and Gubernatorial races across the country and meeting with his political team for real-time updates", White House Press Secretary Sarah Sanders said. 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. Will Trump still be President?
Is the economy strong enough to help the Republicans Tuesday and hurt the Democrats?
This is because Democrats have more seats up for re-election this year, with the Republicans defending only nine.
Democrats are trying to hold 10 Senate seats in states where Trump prevailed in the 2016 election, including Tennessee.
Given Trump's stunning victory in 2016, few were confident in their predictions.
"I would say tone".
If Democrats are to do well, turnout will be key and they will need to mobilise their base in the same way Mr Trump rallied support two years ago.
FILE - People stand cast their ballots ahead of the November 6 election at Jim Miller Park, in Marietta, Ga., Oct. 27, 2018.
"The biggest worry for the White House in terms of re-election is making sure they have a Republican governor in Florida and OH because it just makes organizing those states all the easier", he said. "It's an awakening of the Democratic Party".
Secondly, it could lead to a string of damaging congressional investigations being launched by House committees controlled by the Democrats, on everything from Mr Trump's tax returns to the business interests of his family.
On Twitter, he said the vote "might be the most important of our lifetimes".
Voting will begin on the east coast at 11pm AEST, with early voting figures showing a huge increase in turnout since the last midterm elections in 2014.
If ever there was an off-year election for younger voters to break tradition, this is it. This year they filed a lawsuit to overturn what remains of the ACA and it was backed by the justice department, which said it concurred with the need to end the law's guarantees that insurance companies could not deny coverage to people with pre-existing conditions.
"Everything we have achieved is at stake tomorrow", he said during a blitz of three final rallies.
It's much tighter in the Senate, where the Republicans have only two more.
Democrats boast record diversity on the ballot.
"I think the Senate will be too much of an uphill climb".
In Georgia, Democrat Stacy Abrams would become the first black woman to be elected governor in US history if she defeats Republican Brian Kemp. More men than women have seen financial gains (44 percent to 32 percent) and the rich - well, they are rich.
The political realignment, if there is one, could re-shape U.S. politics for a generation. The demographic shifts also reflect each party's closing argument.
Any move to fire Sessions or his deputy, Rod Rosenstein, would light a fire of controversy in Washington over the president's efforts to control or interfere in Special Counsel Robert Mueller III's investigation into Russian meddling in the 2016 election and its coordination with United States persons toward that end. 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.