Here are five areas of interest Dem lawmakers will focus on after they get their hands on Trump's tax returns

Writing at Bloomberg, Trump biographer Timothy O'Brien -- author of 'TrumpNation" -- laid out a road map for Democratic lawmakers to use when they finally get their hands on Donald Trump's tax returns.

According to O'Brien, who claims he has seen earlier tax returns belonging to Trump and -- now the House Oversight committee chairman Richard Neal (D-MA) has invoked a seldom-used law to demand six years of Trump's tax filings -- the author said much will be revealed.

"Trump has already said he isn’t inclined to release his tax returns in accordance with Neal’s request, so this is certain to ignite a legal battle," O'Brien wrote. "In the interests of good government and the avoidance of financial conflicts of interest in the Oval Office, I hope Congress wins this one. And I know, for a fact, that it’s not true that you don’t learn much from a tax return. As I noted back in early 2016, I have seen Trump’s tax returns, and I think you should too."

According to the columnist, Trump sued him -- unsuccessfully- for libel in 2011 which allowed his lawyers to get access to Trump's taxes.

Using that as a leaping-off point, O'Brien had some suggestions for the Democrats over where to look.

"I think there are five broad categories of disclosure related to his returns that should matter to voters, politicians, and anyone else interested in making sure the White House is conflicts-of-interest free," he wrote before ticking off:

• Income
• Business Activities
• Charitable Giving
• Tax Planning
• Transparency and Accountability

On particular note was O'Brien's comments on transparency, noting it would be a rich vein to mine.

"This may be the most important category of all," he wrote. "Trump is now, arguably, the most powerful and influential man in the world. His tax returns would provide a much clearer picture of potential financial conflicts or pressures that would come to bear on him in the White House. They would also provide a way of monitoring whether the president is more interested in his financial self-interest and deal-making than policy-making."

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