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Large protests are expected Saturday across the country pegged to Tax Day to pressure President Donald Trump to release his tax returns.

This year's Tax Day Marches on Saturday, planned in dozens of cities across the county, are expected to be the biggest political mass mobilization since January's Women's March, which some believe was the largest mass political mobilization ever recorded.

Organizers hope to call attention to the fact that Trump is the first president since Richard Nixon to refuse to release his tax returns, and to prepare for a fight on tax policy.

“We need to see Trump’s tax returns as a matter of transparency. If we’re going into a tax reform debate, we need know if what Trump wants to do is going to benefit himself, since he tends to do things that help him and not necessarily others,” said Wes Shockley, one of the organizers of the New York City march.

And with a big fight on tax reform looming in Washington, progressives are hoping the demonstrations will start to energize grassroots activists about taxes, which is typically seen as GOP turf.

April 15 marked the kickoff of the Tea Party movement in 2009, which sprung up as a reaction to the presidency of Barack Obama. But activists note the anti-Trump “resistance” movement has already been underway for several months.

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Saturday’s rallies — sponsored by a large coalition of mostly left-leaning groups — is unlikely surpass that magnitude, but cities from Denver to Washington, DC are nonetheless expecting tens of thousands.

Trump has said in the past that he can't release his returns because he is being audited by the IRS.

Experts say an audit would not prevent Trump from making his returns public. And a Pew survey from January found almost two-thirds of Americans think Trump has an obligation to release his tax return.

In spoiling for a fight on taxes, progressives are stepping onto turf that has often favored Republicans. Cutting taxes has long been a top priority for the conservative base, while GOP candidates have for decades found success in attacking Democrats for wanting to raise taxes.

RELATED: Who is leaking Donald Trump's tax returns?

But Trump and congressional Republican leaders are promising to take up a major code overhaul soon, and activists are hoping to have as much influence on that debate as they did on the effort to repeal the Affordable Care Act.

“It has sort of been a Republican issue, but people are starting to make the connection about why we pay taxes and what that money pays for,” said Shockley.

With united Republican control of government, Democrats are worried Republicans will cut taxes on the wealthy and corporations and financially suffocate government programs they support.

“Progressives support an amply funded government sector, which means we must pay a lot of attention to tax policy,” said Jared Bernstein, a former top economic adviser to former Vice President Joe Biden.

RELATED: Trump paid $38M in federal income taxes in 2005

Elizabeth Williams, one of the people behind a Tax Day March in Minnesota’s Twin Cities, said organizers have tried to make the march as bipartisan as possible, with a message that she hopes can appeal to anyone.

“It’s about financial transparency in political campaigns and also about a fair tax system that works for everyone,” she said.

This year, taxes are actually due on Tuesday, April 18 because April 15 falls on a weekend and Monday is a federal holiday.