The tornado in Nashville was just one of three to hit Tennessee early Tuesday. Eighteen people died in Putnam County, Tennessee, officials said Tuesday night, after earlier in the day reporting that 19 had died there.
Fatalities were also reported in Wilson, Davidson and Benton counties.
A state of emergency was declared in Tennessee as a result of the damage.
Two of those killed were Michael Dolfini, 36, and Albree Sexton, 33, according to Nashville police.
About 47,000 are without power and dozens of building have collapsed, officials said.
Winds reached at least 155 mph -- EF-3 tornado level -- in Mount Juliet and Donelson.
Some Super Tuesday polling stations were impacted, officials said. Alternative locations will be offered for voters. Tennessee Secretary of State Tre Hargett said residents can call the Division of Elections toll-free at 1-877-850-4959.
A judge ruled that all Tennessee polling stations would have to extend Super Tuesday hours, a Democratic Party spokeswoman told The Associated Press.
Nashville resident James Duncan said sirens started in the middle of the night before "howling" winds moved in.
"Things started hitting the window, and my girlfriend and I shot out of bed and darted for the bathroom. We could hear objects slamming against the building ... it was terrifying," Duncan told ABC News. "They say tornadoes sound like a train ... they were not lying. The feeling in my head from the pressure was insane. I've never felt anything like it. We were shaking."
"Then the power cut out and the noise started to fade," he continued, but he smelled a gas leak and first responders quickly evacuated the apartment to investigate the suspected leak.
"Downed power lines were everywhere," Duncan added. "We had to walk roughly a mile through the debris and wreckage 'til we could get to a zone where a friend could pick us up. I was trying to reach out to friends in the area to see if they were OK. It was chaos. It felt like we were in a bad dream."
Jeremy Vaughn said he spotted "the funnel cloud" and heard what sounded like a train.
He told ABC News he ran down to the first floor of his apartment building, where he huddled with his neighbors, who "were crying and screaming -- we didn't know what was going on."
Outside, Vaughn added, it "looked like a war zone," with the apartment building's roof on the ground, the street littered with insulation and pieces of wood.
In hard-hit Mt. Juliet, 7-year-old Brock Maxwell was rescued by Mt. Juliet Fire chief Jamie Luffman in the aftermath of the twister.
Brock's family took video of the moment Luffman carried him to safety across live wires in the middle of the night.
The twisters left planes decimated at the John C. Tune Airport in West Nashville. No one there was injured and crews from Nashville International Airport will help rebuild, said Doug Kreulen, Nashville International Airport president and CEO.
President Donald Trump in a tweet pledged the federal government's support. Trump said he will travel to Tennessee on Friday.
While the "damage is devastating," residents are already stepping up to help their community, according to Nashville Mayor John Cooper.
He said the website for Nashville volunteer organization Hands On Nashville has crashed three times from so many people trying to sign up to volunteer. More than 5,000 people signed up by Tuesday afternoon, he said.
The damage and power outages shuttered Nashville schools Tuesday and Wednesday.
The mayor posted photos of himself assessing the severe damaging at one Nashville elementary school.
Nashville is the capital of country music and stars have spoken out in the wake of the destruction.
Country music singer Maren Morris tweeted that "the tornado must have missed our block by an inch."
"I am so depleted looking at the damage that has happened to our beautiful city. There are so many people in the streets helping already, though,” she said. “Thinking of those who lost their loved ones + homes."
ABC News' Max Golembo, Rachel Katz, Jordyn Phelps, Erin Calabrese, Victor Oquendo, Christopher Lumsden and Meg Cunningham contributed to this report.