By Steve Holland

ROANOKE, Va. (Reuters) - Donald Trump eagerly injected himself into the Democratic Party's email controversy on Monday, saying the revelations that the party apparatus backed Democrat Hillary Clinton over Bernie Sanders proved his charges that the system is rigged.

Trump, kicking off a three-day campaign swing with his vice presidential running mate, Indiana Governor Mike Pence, returned to his freewheeling style after giving a scripted speech on Thursday accepting the Republican presidential nomination.

During an hour-long event in Roanoke, Virginia, Trump labeled Clinton "low-energy," the same characterization he lobbed at Republican rival Jeb Bush; attacked her running mate, U.S. Senator Tim Kaine of Virginia; and complained about the air conditioning in the hotel ballroom where he spoke.

"I think the ballroom and the people who own this hotel ought to be ashamed of themselves," Trump said.

Trump took particular delight in making light of Democratic disunity as party loyalists gather in Philadelphia this week to anoint Clinton as their nominee, after a week in which Republicans struggled to unify behind Trump at their convention in Cleveland.

Trump waved away Republican disunity as essentially isolated pockets of resistance and made an apparent reference to U.S. Senator Ted Cruz, who was booed off stage in Cleveland when he did not endorse Trump after losing to him in a bitter primary race.

"We had a couple people who probably destroyed their career, but who knows," Trump said. "Look what's going on in Philadelphia. ... We had no riots, no nothing. It was unbelievable. I'll never forget it as long as I live."

On Twitter, Trump added, "Wow, the Republican Convention went so smoothly compared to the Dems total mess."

Trump's strongest words in Roanoke were for Democratic National Committee Chairwoman Debbie Wasserman Schultz, who was forced to resign on Sunday in the fallout over leaked emails showing the committee backed Clinton over democratic socialist Sanders.

The New York businessman said it was proof the "system" is rigged against outsider candidates.

"Debbie was totally loyal to Hillary, and Hillary threw her under the bus," Trump said, adding, "I don't want her covering MY back."

He then launched into a riff about "Hillary Rotten Clinton," a play on her full name, Hillary Rodham Clinton.

(Reporting By Steve Holland; Editing by Jonathan Oatis)