Trump announced his latest legally questionable policy proposal after accepting the endorsement of the New England Police Benevolent Association, a police union representing more than 4,000 law enforcement officials -- an endorsement Trump characterized as a "lifetime improvement award."
"One of the first things I do, in terms of executive order if I win, will be to sign a strong, strong statement that will go out to the country -- out to the world -- that anybody killing a policeman, policewoman, a police officer -- anybody killing a police officer, the death penalty. It's going to happen, OK?" Trump said in brief remarks to the several hundred police union members gathered for the endorsement.
Trump did not explain the legality of mandating a death penalty sentence via executive order. Nineteen states, as well as the District of Columbia, have outlawed the punishment. The death penalty is legal at the federal level and prosecutors can seek capital punishment in some murder cases, including ones involving the killings of state or local law enforcement officials, but it's unclear how an executive action could be used to require this.
Trump's campaign did not immediately return a request for comment.
The proposal comes days after Trump called for barring Muslims from entering the U.S., which was roundly criticized by Republicans and Democrats alike as counter to American founding principles of religious tolerance and potentially illegal.
Beyond the shaky legal grounds, Trump's cop killer proclamation may also not sit well with conservatives who have consistently slammed President Barack Obama for using his executive authority too broadly to implement policy proposals he could not pass through Congress.
In his brief remarks Thursday, Trump touted the role of police officers in the face of terrorist threats, pointing to the actions of local police officers last week in the San Bernardino terrorist attack.
He also called for police to have military-style equipment and vehicles.
"We've got to let our police have the finest equipment and the finest training, and if we don't, we're making a tremendous mistake as a country," Trump said.
Trump also remarked on the heat he has taken in recent days since he called for banning Muslims from entering the United States.
"We have people talking, let me tell you that," Trump said, before remarking that he took even more fire from critics when he announced his controversial immigration proposals, which include making Mexico pay for a wall across the U.S.'s southern border and deporting the estimated 11 million undocumented immigrants living in the U.S.
Outside the event, though, an estimated 200 people gathered to protest Trump's presence.
The protesters' signs shared messages of love and welcoming for Muslims and Syrian refugees -- whose entry to the U.S. Trump also opposes -- with signs that read "Love > Hate" and "Being a Muslim isn't a crime."
The protest was largely organized by a Quaker group, the American Friends Services Committee, and protesters expressed concern that Trump's message was resonating so strongly across the country. Trump's support now stands at 36% of the Republican electorate, according to the latest CNN/ORC poll
Ellen Fineberg, a Portsmouth resident in her early 60s, compared Trump's rhetoric to the rise of Adolf Hitler in Germany in the 1930s.
"Donald Trump is maligning Muslims and there is a history in our world of hate speech," she said. "As someone who is of a Jewish background, I know that hate speech was the beginning for Hitler and for what happened in World War II. And so we need to stand up now and really speak out against this kind of derogatory statements that Trump is making across the board."