Washington, Nov 1 (IANS): In the most racially charged national political ad in 30 years, US President Donald Trump and the Republican Party have accused Democrats of plotting to help Central American "invaders" overrun the nation with police killers, CNN reported on Thursday.

The new spot, tweeted by the President on Wednesday five days before the midterm elections, is the most extreme step yet in the most inflammatory closing argument of any campaign in recent memory.

The ad -- produced for the Trump campaign -- features Luis Bracamontes, a Mexican man who had previously been deported but returned to the US and was convicted in February in the killing of two California deputies.

"I'm going to kill more cops soon," a grinning Bracamontes is shown saying in court as captions flash across the screen reading "Democrats let him into our country. Democrats let him stay".

The ad also flashes footage of the migrant caravan of Central American asylum seekers that is currently in Mexico.

"Who else would Democrats let in?" a caption asks.

The ad recalls the notorious "Willie Horton" campaign financed by supporters of the former President George H.W. Bush's campaign in the 1988 presidential election.

Horton was a man and convicted murderer who committed rape while furloughed under a programme in Massachusetts where Democratic nominee Michael Dukakis was Governor.

The ad has since been deemed as one of the most racially problematic in modern political history since it played into white fear and African-American stereotypes, CNN said.

Reacting to the ad, Democratic National Committee Chairman Tom Perez said on Wednesday night the ad was a sign of desperation and suggested that Trump was losing the argument over health care that is at the centre of the Democratic campaign.

"This is distracting, divisive Donald at his worst," Perez told CNN.

"This is fear mongering... They have to fear monger and his dog whistle of all dog whistles is immigration. This has been Donald Trump's playbook for so long."