Ryan Routh, 58, was ordered by US Magistrate Judge Ryon McCabe to remain in jail without bond pending trial on the two gun-related charges brought against him after his September 15 arrest.
McCabe said prosecutors had documented Routh's effort to "stalk" Trump during a roughly month-long period in south Florida "in an apparent attempt to assassinate him".
Prosecutor Mark Dispoto said the Justice Department will ask a grand jury to bring the more serious attempted assassination charge against Routh, who was handcuffed and shackled at the waist during a hearing before McCabe in which prosecutors detailed some of the evidence collected against him.
Department of Homeland Security officers were out in force near a hearing involving Ryan Routh. (AP PHOTO)
Routh wrote a letter, prosecutors said, months earlier referencing an "assassination attempt" on Trump and a $US150,000 ($A219,400) bounty on the Republican presidential candidate's life.
He also assembled a "sniper's nest" near the sixth hole of Trump's West Palm Beach golf club in an attempt to kill him, Dispoto said, but was stopped 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.
Lawyers representing Routh 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 wore a navy blue jumpsuit during the hearing.
He did not speak.
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.
"This was an assassination attempt on Donald Trump but I failed you," the suspect wrote, according to a court filing by prosecutors.
"I will offer $US150,000 to whomever can complete the job."
Kristy Militello, a lawyer representing Routh, argued that the letter indicated "an intent to fail" at the assassination.
"Perhaps this was more of a publicity stunt than anything," Militello said.
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 that 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.
Trump in a statement on Monday called the charges inadequate, saying "The charges brought against the maniac assassin are a slap on the wrist".
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 United States.
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.
