"I don't know how a person with that cloud over their head could be running for president," Trump said of Hillary Clinton, the Democratic frontrunner in the race for the White House.
Trump said if the allegations are true that Clinton sent classified email from her private server, it's "criminal."
"They destroyed him. He's like a ruined man," Trump said of Petraeus.
Trump argued that Petraeus "did much less than Hillary in terms of information, in terms of importance and in terms of volume," of the classified material leaked.
Trump didn’t reserve all his criticism for Clinton. He moved onto Wisconsin Gov. Scott Walker. He said that the state of Wisconsin is doing "terribly" and accused Walker of being a flip-flopper.
"He was totally in favor of Common Core," Trump said. "Scott Walker changed when he saw he was getting creamed."
Trump said he was retaliating for nasty comments made by a Walker fundraiser.
"Today I read this horrible statement from his fundraiser guy about Trump, and I say, 'Finally, I can attack. Finally,'" Trump said.
"I can't believe I'm in second place," Trump said. "Folks, will you please put me in first place, so I feel better."
Before Trump arrived in Oskaloosa, people waited in the heat for two hours to catch a glimpse of the billionaire mogul and reality television star. Trump's whirlwind week of criticizing Sen. John McCain’s war hero status and giving out Sen. Lindsey Graham’s phone number didn’t seem to deter people from coming to hear him speak.
"One thing I really like is he's not afraid to say what he thinks and stand up for it," said Karen Lambert of Des Moines, Iowa. "I'm tired of the Republicans just caving for pressure and they really need to stand their ground and I think that Trump is someone who will do that."
Lambert is undecided on who her favorite candidate is in the Republican field, but she said she appreciates Trump for not always being politically correct.
"I really feel like the GOP leadership has completely ignored the roots, the grassroots, the base of their party and they're trying to be Democrats and we really need a conservative option," she said.
Carl Drost, a native of Oskaloosa, called Trump, the "epitome of the American dream."
"I want to shake his hand before he gets out of here today," Drost said.
"He's kind of outspoken and I've been president of the school board for a couple of years and sometimes being outspoken is not real positive and that needs to be done to let people know what's going on," Drost said.
Trump didn’t back down from his comments made about McCain, reiterating that he believes McCain has done little for veterans. To highlight how much Trump says veterans love him, he called for a box of letters to be brought to the stage that contained what he called "one day of letters, about a half a foot wide stack."
Of his candidacy, Trump said he was in it for the long haul. He joked that there were other more relaxing things he could be doing, but he wants to be president.
“I love you people. I love Iowa, but it’s hot as hell in this room. I could be doing something else right now,” said Trump, generating laughter in the room.
"I'm going to be there, much to the chagrin of everyone. I never did that before. I never stood at a podium and debated a large number of people and from what I heard, every one of them is going to come at me," Trump said.
Trump said he knows he’ll likely be in the "lion's den."
"I don’t know if I'm going to be good," Trump said of the debate. "Politicians do it every night, but they don’t produce jobs."