The liberal super-PAC American Bridge is launching a new site pushing presumptive GOP presidential nominee Donald Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE to release his tax returns as Democrats seize on the issue heading into the general election.

TrumpReleaseYourReturns.com points to several Republican and Democratic presidential candidates who have released their returns, including 2012 GOP candidate Mitt Romney and current Democratic front-runner Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE.

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The website questions why Trump is "backing off his earlier promises to release his tax returns," citing Trump's remarks in 2012 that it was a "positive" for Romney to release his returns.

Trump has pushed back against calls to release his tax returns, telling The Associated Press this week he doesn't plan to do so before the November election. "There's nothing to learn from them," he said.

He had previously said he couldn't release the returns because he was being audited.

Opponents of Trump such as Romney have speculated the businessman is hiding something in his returns.

"Donald Trump's staunch refusal to release his tax returns makes him less transparent than Richard Nixon, who disclosed his under-audit returns in 1972," American Bridge President Jessica Mackler said in a statement shared with The Hill.

"Voters deserve to know whether or not Trump pays his fair share in taxes, for example, or if he's too cheap to give to charitable causes," she added.

Trump's campaign released a letter from his attorneys in March saying his taxes since 2009 "have been under continuous examination" by the Internal Revenue Service (IRS). Trump said as recently as Sunday that he'd release his taxes "as fast as the auditors finish."

The American Bridge website notes that the IRS issued a statement in February stating that there is nothing preventing Trump from releasing his tax returns.

Clinton, Trump's likely opponent in the presidential election, has signaled she plans to focus on the issue.

"What about his taxes? We'll get around to that, too," Clinton said during a rally in New Jersey on Wednesday. "Why doesn't he want to release them?"