"I'm not saying it was close, but you shouldn't say millions," Cuomo said, citing studies he's read. "It's a gross exaggeration."
The President-elect said in a tweet that in addition to winning the Electoral College, he would have also won the popular vote "if you deduct the millions of people who voted illegally." Hillary Clinton leads Trump in the popular vote by more than two million ballots
Cuomo argued that voter fraud is real, but said studies show the number of occurrences is nowhere near the millions.
Issa then pushed back.
"Chris, I love you," he said. "But you are, in fact, talking about dozens and I'm saying out of 320 million Americans it is a lot more than dozens."
The California congressman also said that "in every district in America, especially those who don't have any kind of ID, people do vote illegally."
The back and forth continued with Issa relenting, momentarily, on the numbers.
"Chris, I love you," Issa repeated. "But you know you have to remember it's about bringing confidence back to the voting process."
Cuomo pressed on, asking, "Why would you say millions of people voted illegally? That doesn't bring back confidence."
"We need to modernize the system and make it accurate and believed by the American people," Issa concluded. "That's all Donald Trump is getting America thinking about."
Democratic congressman Jerrold "Jerry" Nadler appeared on "New Day" during the next segment. CNN's Alisyn Camerota asked for his response to Issa's claims that huge numbers of Americans voted illegally.
"Prove it," Nadler responded. "I don't believe a word Darrell Issa ever says."