>> Democratic debates: 5 memorable moments from Thursday's matchup
“You never know who’s gonna be tough,” he said. “Sometimes the ones I thought would be the toughest, weren’t the toughest.
On Friday, Carter said he believed Trump would have lost the 2016 election without Russian help.
Saturday, Trump called Carter “a nice man,” but “a terrible Democrat.”
“I was surprised he’d make a statement like that,” Trump said. “He’s been trashed by his own party.
“He was not a good president. Look what happened in Iraq, they tied him up in knots.”
>> Jimmy Carter: Trump won election because of Russian help
Trump, who is flying to South Korea after the G20 summit, said he was open to a meeting with Kim.
"Kim Jong Un was very receptive," Trump told reporters. "He responded, so we'll see."
Trump said he would feel "very comfortable" crossing the border into North Korea if he and Kim meet at the demilitarized zone.