With control of Congress and Trump's agenda at stake, early voting hit record levels and turnout appeared high across the country.
All eyes are on the several dozen razor-thin races that will decide if Republicans retain control of the House of Representatives and Senate or if one or both chambers flip Democratic.
All 435 seats in the House, 35 seats in the 100-member Senate, and 36 governorships are up for grabs as Americans cast their ballots.
"I feel confident that we will win," said Nancy Pelosi, the Democratic House minority leader. "It's just a question of the size of victory."
Republicans enjoy a slim 51-49 edge in the Senate and are favored to hang on to their majority since Democrats are defending 26 seats, and Republicans only 9.
But with memories still fresh of Trump's upset victory over Hillary Clinton in 2016, much uncertainty remains, with first results expected to trickle in around 6:00 pm Eastern time (2300 GMT).
Trump embarked on a bruising schedule of rallies over the past few weeks to drive his supporters to the polls.
"The midterm elections used to be, like, boring," Trump told a crowd Monday, November 5, in Cleveland, Ohio. "Now it's like the hottest thing."
Early indications would appear to support this.
"It's a very good turnout," said William Harkins, chief election officer at a polling station in Arlington, Virginia.
According to Michael McDonald of the US Elections Project, 38.4 million Americans cast their ballots early this time compared with 27.4 million in the 2014 midterm.
"I think, like the TV stations have been saying, it's a referendum on the Trump presidency," said Eloisa Alvarez as she voted in Miami.
Below is a collection of live tweets pulled in from several key accounts on the elections.
A Twitter List by rapplerdotcom
Voting in Chicago, James Gerlock, 27, a Republican, said he wanted to see continued growth under Trump.
"I am extremely happy with the economy," Gerlock said. "I just love the deregulation and I just want to keep everything moving, because I'm loving it."
Trump was spending Election Day at the White House and tweeting support for Republicans in the toughest races.
The president has boasted about the economy and hammered away at illegal immigration in the closing days of the campaign as he seeks to retain a congressional majority and salvage his legislative agenda.
He claimed Monday that Democrats wanted to make America "a giant sanctuary city for drug dealers, predators and bloodthirsty MS-13 killers."
Democrats accused Trump of fear-mongering and former president Barack Obama decried his deployment of troops to the border with Mexico as a "political stunt."
Beto O'Rourke, a charismatic Democrat seeking to dethrone Republican Ted Cruz in Texas, said the state is one of "immigrants, asylum seekers and refugees."
"This is our source of strength," O'Rourke said as he voted in El Paso. "No one from outside can stir paranoia or fear that can change that."
The O'Rourke-Cruz race is one of a handful of Senate contests being closely watched and O'Rourke, a former punk rocker, has mounted a surprisingly tough challenge against Cruz in traditionally Republican Texas.
Carlos Gallinar, an El Paso resident, said he planned to vote for O'Rourke because of "the negative energy coming out of Washington and this president."
Other tight Senate races are in Arizona, Florida, Indiana, Missouri, Montana, New Jersey, Nevada, North Dakota, Tennessee and West Virginia.
'Today, we say enough'
Hillary Clinton urged her 23.6 million followers on Twitter to get out and vote.
"For the past two years, we've watched this administration attack and undermine our democratic institutions and values," Clinton said. "Today, we say enough."
"We'll vote for fantastic candidates all over the country – including a historic number of women – who want to raise wages, fight for justice, and help more people get health care."
One race involving a woman is seen as a potential bellwether for how the day may go.
Democrat Amy McGrath, a former Marine Corps fighter pilot, is running neck-and-neck against incumbent Republican Andy Barr in a House race in Kentucky, a state Trump won by 30 points.
Another key House race is in California, where Democrat Harley Rouda is seeking to unseat longtime Republican incumbent Dana Rohrabacher in Republican bastion Orange County.
"I'm ready to ride the Blue Wave," said Barry Rubin, 60, displaying a surfboard that read "Surfers for Harley Rouda."
Carol Reynolds, 83, described herself as a former Republican as she cast her ballot in Laguna Beach.
"I think Trump is awful," Reynolds said. "He destroyed America."
Trump, whose approval ratings are mired in the low 40s, has vowed to his supporters at every rally they will "win, win, win."
But he conceded on Monday the House may slip from his party's grasp.
"We'll just have to work a little bit differently," he told reporters when asked how he'd live with a Democrat-controlled lower chamber.
The party of a first-term president tends to lose seats in off-year elections and Trump's heated rhetoric and the acrimonious political climate has made even some Republicans jittery.
A Trump supporter in Florida was charged recently with sending pipe bombs to high profile Democratic critics of the president and a gunman walked into a Pittsburgh synagogue and shot 11 worshippers dead several days later. – Rappler.com