"See, President Trump? It's not that hard," de Blasio tweeted with a link to his own 2016 tax records.
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"We won't bore you with the endless excuses our president has made. He'll just say this: No one forces any of us to run for public office, but those in public office must be transparent about our finances," the mayor wrote in a post on Medium.
Protesters around the country rallied last weekend to demand the president release his tax returns. Trump and his advisers have maintained that he will not release them because they are under audit, breaking from decades of precedent by past presidents who have made their returns public.
A Quinnipiac poll last month found an overwhelming majority of Americans — 68 percent — think Trump should release his tax returns. Throughout the campaign, the Republican said he wanted to release the taxes but was unable to as the IRS was reviewing them.
Though the tone from Trump advisers shifted after Trump's surprise victory last November, with aides saying the election results show Americans are uninterested in the president's financial situation.
But the issue has been a lightening rod for critics who say Trump's lack of transparency suggests potential conflicts of interest. Republican Sen. Tom Cotton was booed by constituents at a town hall Monday when he relayed Trump's audit explanation when asked about why the president has not made the returns public.