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President Donald Trump has said no one cares about his tax returns, but tens of thousands of people want to prove him wrong next month with massive protests in Washington, D.C. and around the country that organizers hope will be the biggest anti-Trump demonstrations since January’s Women’s March.

Scheduled for April 15, the day federal income taxes are due, The Tax March aims to pressure Trump to release his tax returns, something every president since Richard Nixon has done.

Tax Day also marked the official kick off of the Tea Party movement in 2009. It’s a sign of how much quicker the opposition to Trump has organized itself that this year, the date will see the third or fourth major wave of demonstrations against the new president.

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The protests started the day after Inauguration with the Women’s March, then moved to airports to condemn Trump's travel ban, then took over town hall meetings during last month’s congressional recess.

Like the Women’s March, which started with a Facebook post by a grandmother in Hawaii and snowballed into one of the largest single demonstrations in U.S. history, the idea for the Tax March began on social media.

A single tweet from a comedian and former writer for the Colbert Show, Frank Lesser, is credited with sparking the idea.

Now, dozens of liberal groups and activists have signed on to help organize the Tax March. They are expecting tens of thousands to attend the main march in Washington, D.C. or one of the dozens of other demonstrations in cities across the country.

On Friday, 11 new groups joined the effort, including Common Cause, CREDO, Daily Kos, the Economic Policy Institute, and Public Citizen. They joined major unions like the American Federation of Teachers, organizing groups like MoveOn.org and the Indivisible Project, Sen. Bernie Sanders’ Our Revolution, and the liberal Working Families Party.

“Accountability begins with transparency, and for President Trump, transparency begins with his tax returns,” said Karen Hobert Flynn, the president of Common Cause. “If Donald Trump’s claims that he has nothing to hide are to be believed, then he must begin by releasing his taxes – a low presidential bar that even Richard Nixon met while he too was under IRS audit.”

This week, Trump’s tax returns were back in the spotlight after the White House confirmed the veracity of a portion of Trump’s 2005 tax return first reported on MSNBC’s Rachel Maddow show.

Experts say Trump’s tax returns would provide critical information about his effective tax rate and charitable giving, and could potentially expose conflict of interests he may have with businesses or foreign governments.

Trump has refused to release his returns, claiming he is under audit. The IRS say that should not stop him, and previous presidential candidates have released returns while facing audits, but Trump has claimed, “the only ones that care about my tax returns are the reporters.”

Nearly seven-in-ten Americans say Trump should release his tax returns, according to a recent Quinnipiac Poll, including 65 percent of independents. Just 27 percent said he should not release then.