Hillary Clinton will release her 2015 tax returns by the end of the week, in a move that will put her Republican opponent under further pressure to follow course.

According to multiple reports, Clinton plans to make her most recent tax returns public as her campaign continues to push the narrative that Donald Trump may be hiding something in his own income tax returns by refusing to release them.

Her running mate, Virginia Sen. Tim Kaine, is also expected to release his annual tax returns from the last decade, CNBC reported on Thursday.

Trump has declined to release his tax returns while they are under audit by the IRS, despite the agency's repeated acknowledgement that doing so would not impact their review. In May, Trump campaign chairman Paul Manafort claimed the release of the candidate's taxes "isn't an issue that middle America is interested in."

The Clinton campaign, as well as notable Republicans like Mitt Romney, strongly disagree.

"What is he so afraid of us finding?" the former secretary of state's campaign wrote to supporters in a May fundraising email. "It's time for [Trump] to play by the rules and release his tax returns."

Around the same time, Romney took to Facebook to suggest "there is only logical explanation for Mr. Trumps' refusal to release his returns: there is a bombshell in them."

Clinton and her husband, former President Bill Clinton, released seven year's worth of tax returns last year and paid a federal income tax rate of 35.7 percent in 2014.

Asked about his own tax rate during an interview in May, Trump told ABC's George Stephanopoulos: "It's none of your business."