A lawyer representing the gunman suspected of planning to assassinate Donald Trump has played down his actions as “more of a publicity stunt than anything”.

Ryan Routh, 58, was handcuffed, shackled at the waist and wearing a navy prison jumpsuit when he appeared in court on Tuesday and did not speak (AEST).

So far he has been hit with only two gun-related charges after allegedly hiding in wait on Trump’s Florida golf course with an AK-47 while the former US president was playing a round on September 15.

A prosecutor said the US Justice Department planned to charge Routh with attempting to assassinate a major political candidate, which carried a life sentence if convicted.

Prosecutors told the court Routh wrote a letter months earlier referencing a failed “assassination attempt” on Trump.

The handwritten letter, dropped off at a witness’s home, detailed a $US150,000 ($219,400) bounty on the Republican presidential candidate’s life.

“This was an assassination attempt on Donald Trump but I am so sorry I failed you,” the suspect wrote, according to a court filing by prosecutors.

“I tried my best and gave it all the gumption I could muster.

“It is up to you now to finish the job and I will offer $US150,000 to whomever can complete the job.”

But Kristy Militello, a lawyer representing Routh, argued that the letter indicated Routh had not intended to be successful at the assassination.

“It seems to show multiple times an intent to fail,” she said.

“Perhaps this was more of a publicity stunt than anything.”

Routh’s lawyers unsuccessfully sought his release on a $US250,000 bond, questioning the prosecution’s evidence and arguing that Routh has attempted to aid democracies including Ukraine and Taiwan.

Routh was ordered by US Magistrate Judge Ryon McCabe to remain in jail without bond pending trial on the two gun-related charges.

‘Slap on the wrist’

In a statement on Tuesday (AEST), Trump said the charges brought against “the maniac assassin” were just a “slap on the wrist”.

Prosecutor Mark Dispoto said the Justice Department would ask a grand jury to bring the more serious attempted assassination charge against Routh.

He said Routh had assembled a “sniper’s nest” near the sixth hole of Trump’s West Palm Beach golf club in an attempt to kill the Republican candidate. He was prevented when a US Secret Service agent opened fire after spotting a rifle poking through the fence.

“This was an easy shot,” Dispoto said of Routh’s position, adding that Trump would have arrived in that area roughly 15 minutes later.

Prosecutors said that several months prior to the incident, Routh dropped off at the home of an unidentified civilian witness a box including ammunition, a metal pipe, four phones and a handwritten letter addressed to the “world” that offered a bounty on Trump.

FBI Special Agent Christian Hull, who testified during the hearing, revealed that another letter had been found near Routh’s alleged perch at the golf course. It was addressed to The New York Times.

Hull did not reveal its contents and it was not used as evidence by the prosecution.

Prosecutors also said that when Routh was arrested this month his car contained a handwritten list of dates in August, September and October of places where Trump had appeared or was expected to appear.

They said a search of his mobile phone records showed that the devices had pinged towers near the Trump International golf course, where the incident occurred, and by the Mar-a-Lago resort, where Trump lives.

Routh so far has been charged with possession of a firearm as a convicted felon and possession of a firearm with an obliterated serial number.

He has not yet entered a plea.

Routh fled after being fired upon by the Secret Service agent and was later arrested along a Florida highway, according to a criminal complaint.

US officials have said Routh did not fire a shot during the encounter at the golf course and did not have a line of sight to Trump, who was a few hundred metres away.

The incident came about two months after another gunman wounded Trump on the ear during a campaign rally in Butler, Pennsylvania.

That gunman was shot and killed by the Secret Service.

The pair of incidents revealed the agency’s strains at a time of rising political threats and violence in the US.

Routh, a struggling roofing contractor who most recently lived in Hawaii, had a criminal history. He was a vocal supporter of Ukraine who was interviewed about his quixotic effort to recruit Afghans to fight against Russia’s invasion.

In December 2002, Routh was convicted in North Carolina of possessing a weapon of mass death and destruction. He was also convicted of possessing stolen goods in 2010, according to court records.

-with AAP