The clip, which has more than 5m views, has been denounced as a racist campaign tactic by both Democrats and Republicans alike.
By Lucia Binding, news reporter
Donald Trump has defended a video he tweeted in which the Democrats are blamed for a Mexican migrant who is boasting about killing police officers.
The video, which looks like an ad campaign video, shows twice-deported Mexican immigrant Luis Bracamontes laughing about killing two officers in California, and saying he wished he had killed more.
It is unclear who commissioned or made the video - but it can be seen on the president's Twitter and Instagram page.
The president said the video is tough "but correct".
The clip, which shows Bracamontes smiling as he brags about the killings, has been denounced as a racist campaign tactic by both Democrats and Republicans alike.
It has taken a central role in the US president's closing midterm election argument amid his frenzy of hardline immigration proposals in the final stages of the campaign.
The video is reminiscent of the infamous "Willie Horton" ad about a black man who raped a woman while out of prison on a weekend furlough.
Condemned as racist, it was used in 1988 against Democratic presidential candidate Michael Dukakis, who supported the furlough programme when he was governor of Massachusetts.
In an interview published on Friday, Mr Trump told the Washington Times: "I don't view it as the Willie Horton ad at all.
"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 the two police officers murdered by the Mexican migrant Bracamontes were invited to Mr Trump's 2017 address to Congress.
Bracamontes' conviction and death sentence were chronicled by the conservative Breitbart News.
His snarling, expletive-filled courtroom rant about killing police officers appeared in a little-noticed January 2018 Trump campaign ad.
The US leader tweeted out the video on Wednesday.
The video tweeted by Mr Trump has been viewed more than 5 million times in two days.
Republican Senator Jeff Flake of Arizona, a frequent critic of Mr Trump, said: "This is a sickening ad. Republicans everywhere should denounce it."
Tom Perez, chairman of the Democratic National Committee, called the new ad the "dog whistle of all dog whistles."
He told CNN: "This is distracting, divisive Donald at his worst. This is fear-mongering."