WASHINGTON — Marly Arevalo, 21, has been outside the Supreme Court many times. She lives about 30 minutes away, in Riverdale, Maryland, and often participates in rallies with the immigrant rights group CASA de Maryland.

Monday morning, she stepped inside the Supreme Court building for the first time, to hear attorneys and the justices debate legal standing and executive authority.

The arguments also were about her.

Arevalo came to the U.S. from Guatemala when she was 15 with her mother, father and two siblings. All of them are undocumented. She and her siblings would be eligible for one of President Barack Obama’s immigration policy plans currently on hold due to a lawsuit now before the Supreme Court.

She was one of a number of undocumented immigrants who went to the Supreme Court for Monday’s oral arguments. Most rallied outside the building. A few, like Arevalo, got to see the court in action.

A lawyer representing three undocumented mothers, unable to travel from Texas, received 10 minutes to argue for the Obama policy. The three, in turn, represent nearly 4 million undocumented immigrants who would be eligible for temporary work authorization and relief from deportation concerns under programs Obama announced in 2014.

The court is considering whether Obama overstepped his authority by announcing plans to create the Deferred Action for Parents of Americans and Lawful Permanent Residents program, or DAPA, and expand the existing Deferred Action for Childhood Arrivals program, or DACA — and whether Texas and other states have standing to sue the federal government over them.

Arevalo said she was anxious before the arguments, and left the courtroom unsure of what would happen. If the Supreme Court sides with the federal government, she hopes to be eligible for the expanded DACA program so she can become a social worker.

“I feel more motivated to keep advocating and participating, to fight for my rights and the rights of other families,” Arevalo said.

More than 30 undocumented mothers who would benefit from DAPA participated in a 48-hour fast outside the Supreme Court, praying that the justices see the case the way they did. Maria de Lourdes Reboyoso, a member of the group Mujeres Unidas y Activas, came from San Francisco for the hearing and took part in the fast, although she did not go inside the court. She is undocumented and has four children ranging in age from 19 to 28, two of whom are U.S. citizens.

“I came here because this was very important for me, for my family and for more than 5 million families,” Reboyoso said afterward in Spanish, through an interpreter.

Elia Rosas, who would be eligible for DAPA if it goes into effect, came to Washington from Dallas with her daughters. One of her daughters, Greisa Martinez, an advocate with the group United We Dream, said going inside felt like “walking into history.”

“It was a mix of emotions because we didn’t know whether they were going to approve or deny,” Jocabet Martinez, another of Rosas’ daughters, said. “Either way, we’re still going to fight for mothers and fathers.”

Likely the youngest attendee was Sophie Cruz, a 6-year-old who delivered a letter to Pope Francis during his trip to the U.S. last year about her parents, who are both undocumented. Cruz and her sister are U.S. citizens, so her parents could be eligible for DAPA. She and her mother, Zoyla, attended the arguments.

Sophie Cruz talked at a press conference afterward, after being lifted to the microphone by Jose Antonio Vargas, a journalist and immigration reform advocate who would be eligible for the expanded DACA program. Vargas also attended the arguments.

“We are united by a single mission: We want the same rights for all,” Sophie Cruz said. “I ask the judges to protect us children and all immigrants.”