Donald Trump on Friday pushed back against renewed calls that the presumptive Republican presidential nominee release his income tax returns before the election, and picked a prominent climate change skeptic to help him formulate his energy policy.

American presidential candidates have voluntarily released their tax returns for decades. Democratic presidential front-runner Hillary Clinton and her rival, Senator Bernie Sanders of Vermont, have both released their returns.

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

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

Trump is building out his policy proposals as he pivots to the general election, including tapping experts in various fields.

Among those he has asked for help is U.S. Republican Representative Kevin Cramer of North Dakota, one of the country's most ardent oil and gas drilling advocates and climate change skeptics.

North Dakota has been at the forefront of the U.S. shale oil and gas boom. Cramer endorsed Trump earlier this year.

Trump has asked Cramer to write a white paper, or detailed report, on energy policy, according to Cramer and sources familiar with the matter told Reuters.

Cramer was also among a group of Trump advisers who recently met with lawmakers from Western energy states, who hope Trump will open more federal land for drilling, a lawmaker who took part in the meeting said.

Cramer said in an interview that his paper would emphasize the dangers of foreign ownership of U.S. energy assets, burdensome taxes, and over-regulation. Trump will have an opportunity to float some of the ideas at an energy summit in Bismarck, North Dakota on May 26, Cramer said.

A spokeswoman for Trump's campaign did not comment.

Trump has been light on details of his energy policy so far, though he recently told supporters in West Virginia that the coal industry would thrive if he were president. He has also claimed global warming is a concept "created by and for the Chinese" to hurt U.S. business.

Clinton has advocated shifting the country to 50 percent clean energy by 2030, promised heavy regulation of fracking, and said her prospective administration would put coal companies "out of business."

But the ins and outs of campaigning continue to be a major topic with Trump, who has never held elected office.

On Friday, the billionaire real estate developer, who has often boasted of his wealth, was asked why he had been willing in the past to release his tax information to Pennsylvania and New Jersey officials when seeking casino licenses, even though he was being audited by the Internal Revenue Service (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," he added. "It used to be a secret thing." Trump has said there is nothing voters can learn from his tax filings.

The IRS declined to comment on whether he or any other presidential candidates were being audited.

However, the Trump campaign earlier this year released a letter from his attorneys saying his personal tax returns have been under "continuous examination" from the IRS.

This week, Clinton began calling on her probable Republican rival to release his returns, as she has. Last August, the former U.S. secretary of state posted the past eight years of tax returns for her and her husband, former President Bill Clinton, on her website. Sanders released his 2014 return in April.

Presidential candidates have a long history in the modern era of releasing their tax returns.

"In 1976, Gerald Ford did not release his returns, but he did release some information about his taxes," said Joseph Thorndike, director of the Tax History Project at Tax Analysts, a nonprofit, nonpartisan group that provides tax news and analysis.

"That was the last time that a major party nominee hasn't done it," he said.

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.

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