A New York man has been criminally charged with threatening to kill former US President Donald Trump, who he once referred to as “Hitler.”

Prosecutors said the defendant, Thomas Welnicki, 72, of Rockaway Beach, threatened to do “everything I can” to ensure Mr Trump’s death, and once inquired about Secret Service protection for former presidents and their children.

A federal public defender representing Mr Welnicki did not immediately respond to requests for comment. Lawyers for Mr Trump did not immediately respond to similar requests.

Mr Welnicki was charged with threatening to kill, kidnap, and inflict bodily harm on Mr Trump.

The case was brought as the Republican former president continues to press false claims that widespread voting fraud caused him to lose re-election to Democrat Joe Biden.

A group of Trump supporters assaulted the US Capitol on January 6, 2021, in a failed bid to overturn the election results.

Prosecutors said Mr Welnicki expressed interest in Mr Trump’s demise between July 2020 and December 2021, during and after Mr Trump’s presidency, in several voluntary communications with US Capitol Police and the Secret Service.

Mr Welnicki allegedly told Capitol Police in a July 2020 interview that if Mr Trump lost the election and refused to step down, he would “acquire weapons” and “take him down.”

Prosecutors said Mr Welnicki called the Secret Service around January 4, 2021, threatening to kill Trump and 12 unnamed congressional supporters, and referring to a $US350,000 ($A487,000) bounty.

“I will do anything I can to take out (Trump) and his 12 monkeys,” Mr Welnicki was quoted as saying.

“Tomorrow (Trump) will be in Georgia, maybe I will.”

Mr Trump visited Georgia that day to campaign for two incumbent Republican senators who sought unsuccessfully to win re-election and keep the US Senate under Republican control.

The complaint said Mr Welnicki’s threats continued in the autumn of 2021, when he likened Mr Trump to Adolf Hitler and referred to Mr Trump’s children.

He also allegedly told the Secret Service on December 2, 2021, that “the new Civil war could break out and taking up arms against the government is justified when ballots don’t matter”.

-AAP