Hillary Clinton's running mate, Virginia Sen. Tim Kaine, said Clinton showed her fitness to be commander in chief in Monday night's presidential debate while her opponent, Donald Trump, "told some whoppers."

"Hillary offered details and Donald didn’t. Hillary answered questions and Donald avoided them. Hillary told the truth and Donald told some whoppers," Kaine said today on "Good Morning America." "He just seemed rattled the longer the debate wore on.

"I think it really showed her off as prepared to be commander in chief and president," Kaine said of the 90-minute debate held at Hofstra University in New York.

Voters saw the first fiery moment of the debate when Trump and Clinton discussed the Trans-Pacific Partnership trade deal. Kaine acknowledged that Trump was "on the attack" at the top of the debate but said the Republican presidential nominee failed to provide his "own plans or policies."

"He was attacking on trade but he’s got a real weakness there," Kaine said of Trump. "If he doesn’t like what we’re doing with respect to trade, why is he making all of his products overseas? He’s part of the problem, not part of the solution."

Kaine, who campaigned in Florida on Monday, said Trump was also unable to provide an answer for why he pushed the "birther" controversy suggesting that President Obama was not a U.S. citizen.

"For five years, Donald Trump pushed a bigoted lie that President Obama wasn’t a U.S. citizen, all evidence to the contrary. All the fact checkers said that he was wrong but he kept pushing it dozens and dozens and dozens of times," Kaine said. "When he was asked to explain it by Hillary and by the moderator, Lester Holt, he just didn’t have an explanation."

Kaine suggested that Trump, who admitted in a news conference this month that Obama is a U.S. citizen, "still has to answer" for what Kaine described as the GOP leader's, "bringing us back to the most painful days in our history."

"This is fundamentally a truthfulness and trust issue," he said.

Kaine identified Clinton's best moment in the debate as when she took on Trump for an earlier comment he made about her appearance.

"He tried to change it and say, ‘No, I was talking about her stamina.’ And she said, ‘You go to 120 countries and you sit before a House committee for 11 hours and then you talk to me about stamina,'" Kaine recalled. "The other thing that was great about it is I was watching it on a channel that had a split screen and in that moment, Hillary was going strong and ready for another four or five hours of the debate and Donald Trump was on the ropes and he was out of gas and that was really, really apparent."

Kaine will debate Trump's running mate, Indiana Gov. Mike Pence, next week at Longwood University in Farmville, Virginia. The senator said he has no plans to change his debate strategy against Pence after watching Clinton and Trump spar.

"Our vision is we’re stronger together as a country. The Trump ticket has a different vision," Kaine said. "Donald Trump decided when he ran for president to write a book and he put out that book and it’s called 'Crippled America.' That’s how he sees the nation we’re living in right now. Hillary and I see a very different nation, stronger together, and that’s what we’re going to talk about next Tuesday."