Tim Kaine and Mike Pence are facing off in the only vice presidential debate in the run-up to the US election, clashing over economy, immigration and other issues.

Early on in the debate on Tuesday night, Kaine, Hillary Clinton's running mate, clashed with Pence, Donald Trump's choice for vice president, over reports that the Republican nominee could have avoided paying taxes for nearly two decades.

Pence responded by saying Trump's tax returns showed he used the tax code "brilliantly"

READ MORE: Who are Tim Kaine and Mike Pence?

The vice presidential candidates seemed unlikely to dramatically change the way voters view Trump and Clinton, who met on the debate stage last Monday.

Still, the nationally televised debate promised a spotlight opportunity for the longtime politicians to introduce themselves to Americans, energise party loyalists and potentially sway the shrinking pool of undecided voters.

In a recent Associated Press-GfK poll, more than half of registered voters said they did not know enough about Kaine to venture an opinion about him and about 44 percent said the same for Pence.

Trump and Clinton's campaigns both tweeted continuously as the got under way.

During a rally in Arizona, Trump said the debate would be "a contrast between our campaign of big ideas and bold solutions for tomorrow versus the small and petty Clinton campaign that is totally stuck in the past".

Clinton, campaigning in Pennsylvania, said she had been keeping in touch with Kaine over email about his debate preparations.

"I think America is going to be very impressed and really feel positive about Tim Kaine as our next vice president," she said.

Clinton was widely viewed as the winner of her opening debate with Trump, rattling the real estate mogul with jabs about his business record and demeaning statements about women, and responding to his attacks with calm rejoinders.

New public opinion polls have showed her improving her standing in nearly all battleground states.

Source: Al Jazeera News And Agencies