"The fact is that what she has done is criminal," Trump told CNN's Jake Tapper on Sunday in a phone interview on "State of the Union."
Trump compared Clinton, the Democratic front-runner, to Petraeus, the former top U.S. military official in Iraq and Afghanistan and Central Intelligence Agency director who resigned amid allegations he allowed his mistress access to classified information.
"What she did is far worse than what General Petraeus did and he's gone down in disgrace," Trump said. "What he did is not as bad as what Hillary Clinton did, and it's similar. But it's not as bad. I mean, she got rid of her server, he never did anything like that."
The former general pleaded guilty to a misdemeanor charge of mishandling classified information.
Trump said he hasn't yet turned his attention to Clinton but predicted that when he does: "I will beat Hillary easily."
On another topic, Trump also pointed to a new CNN/ORC poll showing GOP voters' strong interest in how he will perform in the first primary debate on August 6.
"I'm not that surprised because I see the kind of crowd we're getting. We're getting the biggest crowds by far and the biggest ovations," he said.
Trump also hit one Republican primary challenger, Wisconsin Gov. Scott Walker.
"His state is having tremendous difficulty ... because they don't have any money," he said, calling Wisconsin "a catastrophe from an economic and financial standpoint."
And he attacked the Des Moines Register, the leading Iowa newspaper that called on Trump to drop out of the race after his attacks a week ago on Sen. John McCain's war-hero status. "They lie and lie and knowingly lie," he said of the newspaper.
Trump has said he'd consider a third party bid for the presidency if Republicans treat him unfairly, but played down that possibility on Sunday.
"I think they've been really nice over the last few days. They're starting to see what's happening. I mean there's a movement going on. This is more than me. This is a movement going on," Trump said.
"People are tired of these incompetent politicians in Washington that can't get anything done," he said. "They can't make deals. They can't do anything."