Cole Tomas Allen, 31, of Torrance, California, also faces firearms charges.
Allen wore a blue prison jumpsuit at his first appearance in Washington DC federal court, two days after authorities said they foiled an attack at the White House Correspondents' Association Dinner, an annual black-tie gathering of journalists and politicians.
Allen has not yet responded to the allegations.
Seated at the defence table flanked by US Marshals, Allen said he would answer all questions truthfully and that he had a master's degree in computer science.
Prosecutors are seeking Allen's detention while the case moves forward.
Allen left a manifesto with family members referring to himself as the "Friendly Federal Assassin" and discussing plans to target senior Trump administration officials who were present in the hotel ballroom.
Blanche said his targets likely included Trump himself.
White House press secretary Karoline Leavitt on Monday described the Saturday night attack as the third major assassination attempt against Trump, after two attempts on his life in 2024.
She compared the rhetoric in the manifesto to criticism of Trump by his political opponents.
"Much of the manifesto of the would-be assassin is indistinguishable from the words that we hear daily from so many," Leavitt said.
"The entire Democrat Party has made their pitch to voters across the country that Donald Trump poses an existential threat to democracy, that he is a fascist."
Prominent elected Democrats have condemned the shooting.
Allen booked a room at the Washington Hilton hotel, where the dinner took place, and travelled from California to Washington DC by train, officials said.
The shooting on Saturday rattled the press dinner, a prominent event on Washington DC's social calendar, sending attendees scrambling under tables and prompting law enforcement to whisk senior officials out of the room.
Trump, who was set to deliver remarks later in the evening, was rushed off the stage by security personnel after shots were fired.
The suspect allegedly fired a shotgun at a Secret Service agent at a checkpoint inside the hotel before being tackled and arrested, according to authorities.
Video footage Trump posted online showed the suspect sprinting through a hallway outside the ballroom.
US officials have said the suspect was subdued just inside a security perimeter and have touted his takedown as a law enforcement success.
But the incident has revived concerns about the safety of Trump, who survived two assassination attempts during his 2024 presidential campaign, and other US officials.
The Secret Service agent was struck but a tactical vest stopped the shot, and the agent was released from a hospital hours later.
Allen, who authorities said was armed with a handgun and multiple knives, in addition to the shotgun, was also taken to a local hospital to be evaluated following the shooting.
