Trump told the hosts of the MSNBC show Morning Joe that the other contenders "should have been hitting Hillary hard" but did not.
Trump said Vermont Sen. Bernie Sanders let Clinton "off the hook" with his statement that "the American people are sick and tired of hearing about your damn emails."
Clinton grinned at Sanders' exclamation, which followed Clinton's own defense of her use of a private email server for government work.
Trump said Sanders is "losing badly if you look nationally" and as a result, should feel compelled to "go a lot stronger."
Trump, who had promised his Twitter followers he would live-tweet the debate, told the hosts he restrained himself in his comments.
Trump also weighed in on a FOX News poll that shows a tie between himself and Republican candidate Ben Carson. He said he and Carson "get along well," but "we will see if that continues." Carson and Trump will debate again on Oct. 28 at the next Republican debate in Boulder, Colo.
Trump's Republican rival, Jeb Bush, gave his take on the debate on Fox's Fox & Friends and echoed Trump's sentiment on the candidates' reluctance to "go after" Clinton.
"There's [been] an FBI investigation, she hasn't been forthcoming ... I would have taken her to the task on that," Bush said.