Tim Kaine, Mike Pence talk over each other in heated debate

Hillary Clinton's vice-presidential nominee's Tim Kaine has battered his opponent Mike Pence about Donald Trump's tax returns in a heated debate.

The senator slammed Pence for giving his tax returns to Trump to prove he was qualified to be vice president, and yet defended Trump for refusing to publish his own tax returns.

Senator Kaine told Governor Pence that his running mate failed to meet even the "Nixon standard" on releasing his tax returns while running for president.

"Why won't he release his tax returns?" Kaine demanded of Pence.

The debate, long tipped to be a boring affair between two uncharismatic candidates, quickly became heated when Kaine began interrupting Pence as he spoke.

While Kaine came out of the blocks quickly to hammer Trump, Pence's quieter, more measured demeanour gave him the upper hand.

But Pence soon began interjecting Kaine's comments, making the debate unintelligible at times.

"Kaine looks like an evil crook out of the Batman movies," Donald Trump tweeted early in the debate.

Meanwhile, Pence accused Kaine and Clinton of running an insult-driven campaign.

Pence said Trump's insults were "small potatoes" compared to Clinton referring to Trump supporters as a "basket of deplorables".

But Kaine replied by saying Clinton had apologised for her comments, but Trump has not said sorry for any of his comments.

"Donald Trump can't start a Twitter war with Miss Universe without shooting himself in the foot," Kaine said.

Kaine repeatedly attacked Trump on his slurs on Mexican immigrants, women and Senator John McCain.

Trump and Clinton will debate again next week.