(CNN) Americans still haven't seen President Donald Trump's tax returns, but his refusal to release them has prompted his Democratic challengers to get a jump on making theirs public.

So what do we know about them that we don't know about Trump? Plenty.

Vermont Sen. Bernie Sanders , the democratic socialist who's vaulted himself into millionaire status with his books, still prepares his returns himself with his wife, Jane, while Minnesota Sen. Amy Klobuchar has a high-end accounting firm prepare her returns, annotated with statements and prepared with year over year comparisons.

Sanders also took a very on-brand $6,000 tax credit for installing solar panels and an efficient water heater in his home in 2016, as did Massachusetts Sen. Elizabeth Warren -- who earned $234 for installing energy efficient insulation in 2013.

CNN reviewed the tax returns of 2020 Democratic candidates who polled above 2% in the most recent poll and have released returns. (Vice President Joe Biden has not yet released his tax returns.)

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