Race-baiting allegations have mired the homestretch 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US networks have withdrawn President Donald Trump's ad about a cop-killing illegal immigrant.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."
Read more at The BBC