A Utah judge found on Tuesday that prosecutors had presented enough evidence to try a 23-year-old man for murder in the killing of the conservative activist Charlie Kirk last year, and allowed them to seek the death penalty.

The highly anticipated ruling by Judge Tony Graf was a major step forward in one of the country’s most scrutinized murder cases. Shortly after the ruling, the defendant, Tyler Robinson, entered a plea for the first time through his lawyer: not guilty to all charges.

Judge Graf said he wanted to set a trial date at the next hearing, which he scheduled for Oct. 23.

In his ruling on Tuesday, Judge Graf said prosecutors had reached the probable cause standard to bring Mr. Robinson to trial on aggravated murder, which could bring with it the death penalty, as well as all six of the lesser charges they had brought against him, including felony discharge of a firearm causing serious injury, obstruction of justice and witness tampering.

The ruling came nearly a year after Mr. Kirk was assassinated during an open-air speaking event attended by thousands at Utah Valley University in Orem.

During a weeklong hearing in July, prosecutors offered up a barrage of evidence against the suspect, Tyler Robinson, including surveillance videos of Mr. Robinson near the crime scene and text messages in which he confessed to his roommate and lover that he had killed Mr. Kirk because he had “had enough of his hatred.”

The case has been delayed by procedural fights, and Mr. Robinson’s lawyers have questioned DNA and ballistic evidence tying Mr. Robinson to the killing on Sept. 10. But they have not offered a concrete defense, apart from arguing that Mr. Robinson should not face the death penalty.

In August, the defense argued that Mr. Kirk’s assassin had fired just once and hit his “intended target,” and thus had not put anyone else in grave danger. “The shooter did not miss his intended target and did not chamber another round, leaving the spent cartridge in the rifle,” Mr. Robinson’s court-appointed lawyers wrote.

But Judge Graf, in Provo, Utah, rejected that argument. He said prosecutors had established probable cause that Mr. Robinson had knowingly put at least one other person in danger by climbing to a rooftop and firing into a courtyard where Mr. Kirk was taking questions from the audience. That ruling is key because it renders the charge punishable by the death penalty.

Judge Graf’s ruling followed several hours of oral arguments from Utah County prosecutors and Mr. Robinson’s defense lawyers. Dressed in a light gray suit, Mr. Robinson chatted and laughed with two defense lawyers sitting beside him during a portion of the hearing. He did not glance back at anyone in the packed gallery behind him, including his family and Mr. Kirk’s parents and widow, Erika Kirk.

Ms. Kirk was a constant presence at the preliminary hearing in July, and again on Tuesday. She watched the proceedings intently, once bowing forward and clutching her head when one of the lawyers mentioned the gunshot that killed her husband.

Mr. Kirk, 31, founded the conservative youth organization Turning Point USA and was one of the country’s most influential conservative Christian activists. A close ally of the Trump administration, he regularly hosted campus debates where he invited liberal critics to question him in exchanges that often went viral on social media. And he consistently opposed gay marriage and gender transitions, describing the latter as a “middle finger to God.”

Legal experts overwhelmingly expected Judge Graf to find probable cause to proceed to trial. Prosecutors argued that “this isn’t a close case.”

Mr. Robinson’s lawyers had challenged other elements of the prosecution’s case, including that Mr. Robinson had killed Mr. Kirk because of his views, but Judge Graf said prosecutors had established those beyond a probable cause standard as well.

In order for Mr. Robinson to be convicted, jurors would need to find that Mr. Robinson was guilty beyond a reasonable doubt, a much higher standard.