"Trump cannot relax ... the very techniques that got him this far may stop him from closing the deal, and that's why I think he's got to really reflect on what he's doing," the former GOP presidential candidate and Speaker of the House said late Monday on Fox News.
"For Trump to get sucked into this at a personal level, tweeting about or repeating a tweet, technically, about Mrs. Cruz is just utterly stupid," Gingrich told host Sean Hannity.
Gingrich suggested Trump's sharing an unflattering image of Heidi Cruz — comparing her to his wife Melania, a former model — could hurt the businessman, who is struggling for support among female voters.
Trump's retweet also signaled "instability" to those who might have warmed to possibly supporting the outspoken businessman as the GOP nominee, Gingrich said, noting that it also "energized" rival Ted Cruz.
Trump's campaign has defended itself by pushing back on an ad from an anti-Trump super-PAC featuring his wife Melania posing nude for a 2000 British GQphoto shoot that sparked the flare-up. Team Trump has claimed the billionaire was simply responding to the ad and denied that any "attack" on Heidi Cruz had taken place. Ted Cruz has denied any connection to the ad or the group behind it.
Gingrich, who was recently at a meeting of Republicans in Washington discussing with Trump the front-runner's path to the nomination, suggested that Trump could have come across more presidential if he had simply dismissed the "truly stupid" ad.
"The answer was simple: Melania was a supermodel on a professional shoot for a very famous magazine. He's very proud of her and she'll be a very beautiful first lady," Gingrich said. 
"Instead, he gets down into this mud that I think really has hurt him. I'm not sure anybody in the Trump campaign understands yet what a big mistake this is," Gingrich said. 
"They can't keep doing this stuff and think that they're going to get the nomination," Gingrich added, noting Cruz has a path to block Trump from capturing the needed 1,237 delegates to secure the nomination.