(CNN) The key line in the Washington Post's new reporting on President Donald Trump upping the pressure on special counsel Robert Mueller amid his increased agitation over the Russia probe is this one: "He has told aides he was especially disturbed after learning Mueller would be able to access several years of his tax returns."

Man. The tax returns. Again.

Ever since he first became a candidate for president, Trump has steadfastly refused to release even a year's worth of returns. That makes him the first candidate for president since World War II not to do so.

Trump's decision was a regular talking point for Hillary Clinton during the 2016 campaign.

"So you've got to ask yourself, why doesn't he want to release them?" Clinton said in May. "Yeah, well, we're going to find out." At the first general election debate, Clinton again hit Trump on his failure to disclose his tax returns. "Maybe he's not as rich as he says he is," Clinton suggested. "Maybe he's not as charitable as he claimed to be." (Read David Fahrenthold for a detailed analysis of Trump's charitable giving .)

Trump's answer throughout the campaign was that he was currently under audit by the Internal Revenue Service and he had been advised not to release his taxes until that audit ended.

But, there were then -- and are now -- problems with that logic.

Second, while Trump has repeatedly cited the audit as the reason for not releasing the returns, that explanation has wavered at times. In the first general election debate with Clinton, Trump said this: "I don't mind releasing...I'm under a routine audit and it will be released as soon as the audit is over." So far, so good right? Then he added: "I will release my tax returns against my lawyers' wishes when she releases her 33,000 emails that have been deleted."

In January, White House senior counselor Kellyanne Conway drifted even more off message on the tax return question. Here's what she said

"We litigated this all through the election. People didn't care," she said. "They voted for him, and let me make this very clear: Most Americans are very focused on what their tax returns will look like while President Trump is in office, not what his look like. And you know full well that President Trump and his family are complying with all the ethical rules, everything they need to do to step away from his businesses and be a full-time president."

She later backed off the suggestion he would never release them but the damage was done.

Trump, at some point in the early days of his presidential campaign, made the decision that releasing his tax returns was more politically fraught than not doing so. That whatever is in the returns carries a higher political price than the incoming he would take over his refusal to release them.

But, time and again, the national conversation turns to the tax returns -- because they are simply the best way to understand the complexities of Trump's wealth. What would we find out if Trump released his taxes -- even one year's worth? This list, from CNN's Jeanne Sahadi , is super helpful in answering that question:

What his income was for that year? How much did Trump deduct? How much he paid in taxes? If he has any foreign bank accounts Whether he has paid taxes to a foreign government Profits and losses from his businesses How -- and whether -- Trump would benefit from his administration's proposal on tax reform.

In short, we'd learn a lot. Which runs directly counter to Trump's longtime assertion that the financial disclosure forms, which he has filed, are much more important than a tax return anyway.

"I released the most extensive financial review of anybody in the history of politics," Trump told ABC in September 2016 . "It's either 100 or maybe more pages of names of companies, locations of companies, etc., etc., and it's a very impressive list, and everybody says that. ...You don't learn much in a tax return."

And yet, according to the Post report, Trump keeps growing more and more agitated at the idea of Bob Mueller laying his hands on the returns.

Why? We don't know. But that Trump was spurred into action -- the collecting of negative information on Mueller and his investigators -- by the news that the special counsel might be able to gain access to his tax returns is deeply telling. And worrisome for Republicans.