"Trump is a guy who promises a lot. But you might have noticed, he has a way of using the same two words right after he makes his biggest, 'hugest' promises. ... 'Believe me,'" Kaine said. "'There is nothing suspicious in my taxes, believe me.'"
"He never tells you how he's going to do any of the things he says he's going to do. He just says, 'believe me.' So here's the question. Do you really believe him?" Kaine said, as the crowd chanted "not one word."
Last week, GOP members questioned Democratic presidential nominee Hillary Clinton's trustworthiness, often times citing her FBI investigation concerning emails from her tenure as secretary of State.
Kaine took the opportunity to explain why he trusts Clinton in front of a jam-packed Wells Fargo Arena.
"She's consistent. She has battled to put kids and families first since she was a teenager — in good times and bad, in victory and defeat, in and out of office, through hell or high water," Kaine said.
"And with Hillary, it's not just words; it's accomplishments," he added. "She wasn't afraid to stand up against thugs and dictators and was a key part of the Obama national security team that decided to go to the ends of the earth to wipe out Osama bin Laden."
Clinton chose Kaine as her running mate last Friday.
The Virginia senator is expected to help Clinton's campaign by further appealing to the Latino vote, since he speaks fluent Spanish. Kaine also brings foreign policy experience, as he currently serves on the Senate's Armed Services and Foreign Relations committee.
Kaine incorporated some Spanish in his Wednesday speech, when he talked about his missionary trip to Honduras.
In a Sunday interview with "60 Minutes," Kaine referred to himself as a "utility player," or someone who can take on a number of different tasks.
"I've been a lieutenant governor and governor and now in the Senate serve on the Armed Services and Foreign Relations committee," he said. "I've climbed, and I haven't missed a rung on the ladder."