Race-baiting allegations have mired the home stretch of the US mid-term elections, turning it into one of the ugliest campaigns in recent times.
US networks have withdrawn President Donald Trump's ad about a cop-killing illegal immigrant.
Meanwhile, racist automated calls targeted prominent African-American candidates in Florida and Georgia.
Control of Congress is up for grabs in Tuesday's poll, which is being seen as a referendum on Mr Trump.
His ability to govern in the final two years of his term will hinge upon the outcome of votes for all 435 seats in the House, and 35 of the 100 Senate seats.
The Republican president - who has been holding barnstorming rallies nationwide, even though he is not up for re-election this year - campaigns in Ohio, Indiana and Missouri on Monday.
In the weeks leading up to the polls, Mr Trump has escalated his rhetoric about his opponents and divisive issues such as immigration, warning voters against Democratic "socialism" and "an invasion" of criminals from the Central American migrant caravan.
His sharp language has appeared to energise conservative voters, but critics have condemned Mr Trump's tactics as fear-mongering.
On Monday, Facebook, NBC and even the president's favourite network, Fox News, announced they would stop broadcasting a 30-second ad paid for by his campaign.
The clip falsely claimed Democrats let into the US an undocumented Mexican immigrant who murdered two California sheriff's deputies in 2014.
The president last week tweeted the clip, but CNN refused to air it at the weekend, calling it "racist".
Asked about the ad on Monday, Mr Trump told a journalist: "A lot of things are offensive. Your questions are offensive a lot of times."
What you need to know about mid-terms:
What about the racist robo-calls?
Automated phone calls in Florida and Georgia have dragged an already toxic political campaign to new lows, targeting two candidates who could become the first African-American governors of those states.
One message falsely claiming to be from US celebrity Oprah Winfrey called Stacey Abrams in Georgia "a poor man's Aunt Jemima" among other racial slurs, referencing a controversial image of a black woman depicted as a slavery-era "mammy" figure.
Calls in Florida targeting Andrew Gillum featured a background of jungle and chimpanzee noises.
They came days after Agriculture Secretary Sonny Perdue, speaking in Florida, described the election as "cotton-pickin' important" - a term with overtones of slavery.
According to the Wesleyan Media Project, no other US general election in the last decade has seen close to so many attack ads as this one.
Are voters energised?
President Trump is galvanising supporters by arguing that a Democratic takeover of Congress would trigger an influx of illegal immigrants and a crime wave.
The president has also been warning the Democrats will destroy a healthy US economy if they win the keys to power.
Most Democratic candidates have tended to avoid directly confronting the president, focusing instead on "kitchen table" issues such as healthcare and economic inequality.
The party hopes the president's hard-line rhetoric will help them win over younger voters, suburban moderates and minorities to the polls.
The Democrats have rolled out their biggest gun: former President Barack Obama, who travelled to Virginia on Monday to get out the vote for its candidates.
"The character of this country is on the ballot," he said.
What is happening with turnout?
Turnout is traditionally low in the US mid-terms, with the 2014 election seeing a post-war record low of just 37%.
But analysts say a sharp rise is likely this year.
Some 34.3 million people have already voted and the real number is probably higher, according to the US Elections Project, a University of Florida-based information source. The figure in 2014 was just 27.5 million.
In Texas, early voting has exceeded the entire turnout in 2014.
However, thunderstorms are forecast for Tuesday along the eastern coast and snowstorms in the Midwest, which could dampen turnout.
What do pollsters predict?
Pollsters say Democrats may win the 23 seats they need to take over the House of Representatives, and possibly 15 or so extra seats.
However, the Democrats are expected to fall short of the two seats they need to wrest control of the Senate from Republican hands.
Governors are also being chosen in 36 out of 50 states.
The first polls close at 23:00 GMT (18:00 EST) on Tuesday.