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.
The 53-second video released Wednesday evening remains pinned to the top of the president's popular Twitter feed.
The ad also showed a clip of the migrant caravan heading toward the USA border from Central America with the caption "Who else would Democrats let in?"
Trump, whose administration takes a hard-line approach to legal and illegal immigration, has focused on the caravan to drum up support for Republican congressional and gubernatorial candidates ahead of Tuesday's congressional elections.
Some Democrats, including Robert Reich, who served as secretary of labour in the Clinton administration, compared the spot to the 1988 "Willie Horton" ad, which supporters of Republican then-candidate George H.W. Bush used to great effect against Democratic rival Michael Dukakis.
In an interview published on Friday, Mr Trump told the Washington Times: "I don't view it as the Willie Horton ad at all". The president says it's just an ad about a "bad guy" who twice entered the country illegally and committed crimes.
Bracamontes' conviction and death sentence were chronicled by the conservative Breitbart News.
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In Georgia, Democrat Stacey Abrams, former minority leader in the state House of Representatives, is seeking to become the first black woman to become governor of a US state.
In the video he says he's killed cops and he's "gonna kill more cops soon".
Bracamontes, who is from the Mexican state of Sinaloa, came to the US illegally in June 1996, during Democrat Bill Clinton's administration.
Luis Bracamontes glares at the jury as the verdict is read in the killing of two law enforcement officers in Sacramento Superior Court, Feb. 9, 2018. "Democrats let him stay". It includes scenes of a migrant caravan moving toward the USA, warning ominously, "Who else would Democrats let in?" and suggesting that more violence would soon penetrate the border.
Trump has scheduled 10 campaign rallies over the next five days in eight states - Florida, Georgia, Indiana, Missouri, Montana, Ohio, Tennessee, and West Virginia.
In Arizona, two women - Democratic Rep. Kyrsten Sinema and Republican Rep. Martha McSally, a former air force fighter pilot - are battling to replace Senator Jeff Flake, a Trump critic who is not running for reelection.
Tom Perez, chairman of the Democratic National Committee, called the new ad the "dog whistle of all dog whistles".