Washington (CNN) Two former friends of Sen. John McCain say his daughter Meghan McCain's pointed criticism of President Donald Trump in her speech at the Arizona Republican's memorial service on Saturday reflected the character of her late father.

"She is her father's daughter," South Carolina GOP Sen. Lindsey Graham said Sunday on CNN's "State of the Union." "If you say something bad about her dad, you will know it, whether you're the janitor or the President of the United States. She is grieving for the father she adored. I think most Americans understand that."

Former independent Connecticut Sen. Joe Lieberman, who joined the program with Graham, said he was a "proud uncle" watching McCain's speech at the memorial service.

"She was direct ... the way John was," Lieberman said.

In her remarks on Saturday, McCain contrasted her father's legacy with Trump's rhetoric, saying, "The America of John McCain has no need to be made great again, because America was always great."

The comment received applause at the service and highlighted the broad differences between McCain and Trump, who was not invited to attend.

Graham and Lieberman were two of McCain's top allies in the Senate and his close friends -- they were often called the "three amigos" -- and their praise for his daughter came after she posted a photo of herself with them online reflecting that bond, calling the two senators her "uncles forever."

During Sunday's interview, the pair looked back fondly on their time with McCain and their travels together.

"His own attitude, the way he faced death with characteristic fearlessness and perspective, really comforted me," Lieberman said. "But you know, we're going to miss him. He's irreplaceable."

Graham, who offered an emotional tribute to McCain last week on the Senate floor, predicted McCain would be remembered as a cornerstone in Senate history.

"John loved the Senate," Graham said. "He was a man of the Senate. He loved the institution. When you write the history of the Senate, he's going to be in the first chapter."

Later in the interview, Graham said the responses to McCain's death showed the appreciation people had for military service, and he outlined a "formula" for those who wanted to engage in public service.

"Work hard, know what you're talking about, and people will listen to you," Graham said. "Pick a cause worthy of a good fight. Fight hard, and be willing to forgive, and ask for forgiveness. And above all else, when you make a mistake, admit it, repeat, repeat until you die, and the next thing you know, the nation will hold you up."