WASHINGTON – President Donald Trump, who has yet to release his tax returns, went after Sen. Bernie Sanders on Tuesday night after the 2020 Democratic presidential challenger released returns that showed him to be a millionaire.

In a series of tweets, Trump first attacked Sanders and later predicted that the democratic socialist, along with former Vice President Joe Biden, would emerge as the two "finalists" against him in the 2020 election. Biden has yet to officially enter the race

"I believe it will be Crazy Bernie Sanders vs. Sleepy Joe Biden as the two finalists to run against maybe the best Economy in the history of our Country (and MANY other great things)!" Trump wrote on Twitter. "I look forward to facing whoever it may be. May God Rest Their Soul!"

It's not the first time Trump has chimed in on the crowded field of Democrats who have entered the race for the White House in 2020. He's attacked Biden's physical contacts with supporters, Sen. Elizabeth Warren's heritage and Beto O'Rourke's hand movements while talking.

More:Who is running for president in 2020? An interactive guide

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This was, however, the first time the president has weighed in on a challenger's tax returns.

"Bernie Sanders and wife should pay the Pre-Trump Taxes on their almost $600,000 in income," Trump wrote on Twitter. "He is always complaining about these big TAX CUTS, except when it benefits him. They made a fortune off of Trump, but so did everyone else - and that’s a good thing, not a bad thing!"

On Monday, Sanders released 10 years of tax returns that revealed he and his wife, Jane, earned more than $550,000 in 2018, including $133,000 in income from his Senate salary, and $391,000 in sales of his book, "Our Revolution: A Future to Believe In."

The filings also show he raked in more than $1 million in both 2016 and 2017.

The documents show that Sanders, who throughout his career has called for an economy and government that works for everyone and not just the 1 percent, is among the top 1 percent of earners in the U.S. According to the liberal-leaning Economic Policy Institute, families in the U.S. earning $421,926 or more a year are part of this group.

After Trump's tweets, some on social media pointed out that Trump has yet to release his tax returns, going against decades of presidents who have made it a traditional act of transparency. The president has claimed he hasn't allowed the public to see his finances because he is under audit.

"How much did you pay in taxes @realDonaldTrump?" Richard Painter, a former White House ethics lawyer under President Barack Obama, wrote on Twitter. "Show us the tax returns. It’s time to put up or shut up."

Contributing: Associated Press