But with turnout a key unknown factor and pollsters still unsure about the effect of Trump's maverick style on voters, both parties admit that they may be in for a surprise.
That is reflected in the early vote totals for these midterms, which are high.
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.
As Election Day drew near, Democrats were increasingly confident, predicting they would pick up at least the 23 seats needed for the majority on the strength of voter enthusiasm, robust fundraising and unusually fresh candidates.
However, after an election campaign marred by extremist violence, opponents called the polls a chance to curtail a president they accuse of deliberately stirring racism and social division in the search for votes.
Christine Giroux exits the voting booth at Transit Town Hall after casting her vote.
And the Democrats also are "ticked off" that Robert Mueller hasn't issued a special counsel report that would nail Trump. In 2014, some 22 million people voted early, a number that pales in comparison with this year's midterms.
Hitting three states in one day, with a final late rally in Cape Girardeau, Missouri, Trump hammered home his boast that "the Republican agenda is the American dream".
Unlike 2016, however, Democrats have historical trends on their side.
"The survival of the country is going to depend on this election", he said as another man stopped for a moment to argue.
President Trump and congressional GOP leaders will see holding the House as validation of their approach to governing. "I wouldn't say it's as important as '16, but it's right up there". "I just love the deregulation and I just want to keep everything moving, because I'm loving it".
Besides stymieing the Trump agenda, a Democratic victory in the House could allow Democrats to launch multiple committee investigations of the Trump administration and possibly even impeachment proceedings. His current approval rating among Republican voters has reportedly touched a whopping 89 percent. On issues regarding the welfare state and economic inequality, they stand much closer to the Democrats.More news: QuickE: Nihalani Files Plea Against CBFC Cuts; Mission Mangal Cast
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"The midterm history is pretty stark in that the president's party usually loses ground in the midterms and it is usually a question of how much ground they lose", said University of Virginia analyst Kyle Kondik.
"This is our source of strength", O'Rourke said as he cast his ballot in El Paso.
Results from the West will begin to roll in, and there are key Senate races to watch in Montana and Nevada. O'Rourke has drawn enthusiastic support from people in Texas' major cities, while Cruz has been doing better in the state's massive rural areas. Let's see what happens, ' Trump said. But if the GOP loses its majority it will need to to go on defense to protect Trump.
The Senate consists of 100 members, two from each state.
"What it means to be Republican is being rewritten as we speak", Fleischer said. "Which is a big difference", she said.
Sean Trende of RealClearPolitics also had some possible good news for the GOP, noting, "There really are believable scenarios that don't require Republicans to win districts that they have written off. Republicans have to catch some breaks, but they don't have to catch breaks in ways that shock and surprise us".
A crowd of people wait in line at a polling center in Utah. "Democrats have to win back the House and stop Trump". 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.
FILE - President Donald Trump acknowledges the crowd at a rally in Chattanooga, Tenn., Nov. 4, 2018.
But as he visited IN on Monday he conceded the House may slip from his party's grasp.
Larry Sabato's Crystal Ball published its final predictions on Monday, which included Democrats forecast to net 34 House seats and capture control of the lower chamber.
Voter turnout in Pinellas County after the Election Day morning rush is favoring Republicans. In the wake of Clinton's defeat to Trump, some wondered whether he could have won in her place. For Gregoire and Kanter - and for voters across the country - the election represents something far greater than whatever Senate and House races appear on their ballots.
In Georgia, Democrat Stacey Abrams is hoping to become the first African American woman to be elected governor in the US.