Trump says his tax rate is 'None of your business'

US Republican presidential candidate Donald Trump pushed back on Friday against renewed calls for him to release his tax returns before the election, saying the rate that he pays is "none of your business."

Trump, who has all but locked up the Republican Party's nomination for the Nov. 8 presidential election, has said the Internal Revenue Service is auditing his returns and he wants to wait until the review is over before making them public.

"It should be and I hope it's before the election," Trump told ABC's "Good Morning America."

Trump, a billionaire real estate developer who has boasted of his wealth during the campaign, was asked why he had been willing in the past to release his taxes to Pennsylvania and New Jersey officials when seeking casino licenses, even though he was being audited IRS.

"At the time it didn't make any difference to me. Now it does," Trump said.

Pressed on what tax rate he pays, Trump refused to say.

"It's none of your business," he said. "Before 1976, people didn't do it. It used to be a secret thing," he added.

U.S. presidential nominees have voluntarily released their tax returns for decades.

Democratic presidential front-runner Hillary Clinton and her rival, Bernie Sanders, have both released their returns. Clinton began calling this week on Trump to do the same. Sanders released his 2014 return in April, while former first lady Clinton posted the past eight years of her and her husband's tax returns on her website in August.

Trump has said there is nothing voters can learn from his tax filing. Tax filings show sources of income, both from within the United States and other countries, as well as charitable giving, investments, deductions and other financial information.

Trump said his company was "clean."

"I don't have Swiss Bank accounts, I don't have offshore" accounts," he said.

The 2012 Republican presidential nominee, former Massachusetts Governor Mitt Romney, has been scathing in his criticism of Trump and said this week it was "disqualifying" for a nominee to refuse to make his tax returns public.

"There is only one logical explanation for Mr. Trump's refusal to release his returns: there is a bombshell in them," Romney said in a Facebook post on Wednesday.

(Reporting by Megan Cassella and Susan Heavey; Editing by Doina Chiacu and Frances Kerry)