Watching former State Department staffers refuse to testify during a congressional hearing about Hillary Clinton's emails was "horrible" and demonstrates the culture of corruption the former secretary of state embodies, Donald Trump charged on Tuesday.

Speaking to voters in Des Moines, the Republican presidential nominee hammered Clinton for displaying an unprecedented "attitude of arrogance," instructing aides of hers to destroy devices that contained her emails with hammers and having a security firm bleach her private server after receiving a congressional subpoena.

"Now the people who destroyed the emails are pleading the Fifth Amendment in front of Congress today," Trump told the crowd at his mid-afternoon rally. "Have you seen what's going on in Congress?"

"'Fifth Amendment, Fifth Amendment, Fifth Amendment,'" he said, mimicking former aides to Clinton who repeatedly invoked their rights against self-incrimination during a House Oversight Committee hearing on Tuesday.

One aide, former State Department IT specialist Bryan Pagliano, declined to attend the hearing despite being subpoenaed to testify. Pagliano previously spoke to FBI officials under immunity about Clinton's private email practices and the security of her server.

"Anyone watching today's hearing knows that Hillary Clinton's actions are far more corrupt than we ever imagined," Trump said. "This is far bigger and a far bigger scandal than Watergate ever was."

By contrast, Trump said that "with Watergate we had justice."

"We had a Justice Department that went after the people," he told his supporters. "Here's something — we've never seen anything like it, folks."