Democratic nominee Hillary Clinton released her 2015 tax returns Friday, thereby increasing speculation about why Republican nominee Donald Trump still refuses to release his own. Hillary and Bill Clinton's tax returns show that they earned $10.6 million last year, paid an effective federal tax rate of 34.2 percent and donated 9.8 percent of their income to charity. The Clintons have now publicly released their tax returns dating back to 1977.

Trump's decision not to release his tax returns breaks with a practice carried out by presidential candidates since the 1970s. During the 2012 presidential race, the businessman called on Republican nominee Mitt Romney to release his tax returns, which Romney eventually did – to criticism of his tax rate. In turn, Romney suggested this February that Trump could be hiding a "bombshell" in his tax returns.

Trump maintains that he will not release his tax returns because he is being audited by the Internal Revenue Service. However, continuing to hold out could fuel more theories about what his real motivations are.

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Jenna Johnson of The Washington Post highlights the irony of Trump's habit of provoking questions about the closets of Clinton, Romney and President Obama. She asserts, "Trump, in building a wall around his records, is setting a new standard for secrecy for modern-day candidates." Johnson argues that Trump likely realizes that he will face political consequences if the public learns of the contents of his tax returns.

Similarly, The New Yorker's Jeffrey Toobin points out that IRS policy does not prevent Trump from releasing tax returns while under audit, leading him to assert, "The main risk of disclosure is political rather than legal."

Alexis Simendinger at RealClearPolitics discusses Clinton's actions Friday in the context of her broader approach to defeating Trump. "Clinton's campaign strategy is not to kick back and count on the real estate mogul to implode, Democrats said, but rather to press a multi-faceted and reinforcing narrative that Trump is a loose-lipped, self-involved hazard to the economy and national security, including among potential GOP skeptics who never imagined casting a presidential ballot for a Democrat, a woman or a Clinton," writes Simendinger.