Donald Trump Jr. on Lewandowski Firing: 'Things Had to Change'

— -- Donald Trump Jr. said today "things had to change" in his father's campaign before the general election, explaining Monday’s firing of campaign manager Corey Lewandowski.

"No, he didn't see this coming," he said of Lewandowski, adding that he wasn't physically escorted from the building. Trump Jr. said he was in the room when Lewandowski was fired. "There was nothing malicious or vicious about it."

“There does have to be a transition to the general" election, Trump Jr. said on "Good Morning America."

Lewandowski had led Donald Trump Sr.'s campaign since the launch of his presidential bid. But a rough stretch and low fundraising numbers have plagued the campaign since Trump became the presumptive nominee in early May.

"I think there’s also time to move on," Trump Jr. said. "Those are the tough decisions you have to make when you're running for president."

A senior aide to the candidate told ABC News that Lewandowski's firing "was a long time coming" and that the campaign "need[ed] to get everyone on the same page."

Lewandowski told CNN Monday he didn’t know why he was fired.

"I don't know. I don't know the answer to that. But what I know is what we've been able to achieve in this election cycle has been historic," he said.

Trump Jr. said that his father plans to announce his vice presidential pick at the GOP convention in July.

He also said that a man who allegedly tried to grab a police officer's gun last night to assassinate his father "of course" concerns Trump Jr., but he praised law enforcement and said, "You can only plan for so much."