Donald Trump says a video he tweeted showing a Mexican migrant in a courtroom smiling and boasting about killing police officers is tough "but correct".
The video, which Democrats and Republicans have denounced as a racist campaign tactic, has taken a central role in the president's closing mid-term election argument.
It comes amid a frenzy of hardline immigration proposals pushed out by Trump in the final stages of the campaign, including sending troops to the Mexican border and revoking birthright citizenship.
The ad is reminiscent of the infamous "Willie Horton" ad about a black man who raped a woman while out of prison on a weekend release.
The ad, condemned as racist, was used in 1988 against Democratic presidential candidate Michael Dukakis, who supported the release program when he was governor of Massachusetts.
"I don't view it as the Willie Horton ad at all," the president told the Washington Times.
"I think it's just an ad where somebody is a bad guy, came in illegally, twice, and we can't do that."
The widows of two police officers the Mexican immigrant murdered were invited to Trump's 2017 address to Congress.
The conservative Breitbart News chronicled Luis Bracamontes' conviction and death sentence.
Bracamontes' snarling, expletive-filled courtroom rant about killing police officers appeared in a little-noticed January 2018 Trump campaign ad. The president tweeted the video on Wednesday.
The immigrant living in the country illegally sports a shaved head and long chin hair.
He speaks in accented English in the video, and his words are clear: He has killed police officers and he is "gonna kill more cops soon".
The video alleges without evidence that Democrats were responsible for allowing Bracamontes into the US.
The twice-deported immigrant from Mexico was sentenced to death in California for the 2014 killings.
"Illegal immigrant, Luis Bracamontes, killed our people!" the video says, adding: "Democrats let him into our country ... Democrats let him stay."
It includes scenes of a migrant caravan moving towards the US, warning ominously: "Who else would Democrats let in?" and suggesting more violence would soon come across the border.
"This is a sickening ad. Republicans everywhere should denounce it," said Republican senator Jeff Flake, a frequent Trump critic.
His Republican colleague Carlos Curbelo, seeking re-election in a Democratic-leaning south Florida district, said he had not seen the video but criticised Trump's use of it.
"I can tell you that it's definitely part of a divide-and-conquer strategy that a lot of politicians, including the president, have used successfully in the past," Curbelo said.
"I hope this doesn't work."
Tom Perez, chairman of the Democratic National Committee, called the new ad the "dog whistle of all dog whistles".
"This is distracting, divisive Donald at his worst," Perez said. "This is fearmongering."
