For all the attention given to the felonies committed by Michael Cohen and Paul Manafort, the Mueller investigation still hasn’t shined a light on the central mystery surrounding Donald Trump's presidency thus far: Just what, if anything, he owes to Russia. If he has no Russian connection, why is the president so deferential to Russian President Vladimir Putin and so intent on silencing Russian accusers like former CIA Director John Brennan? There could be innocent explanations, of course, but many Americans, including some Republicans, are puzzled.

The answer may lie inside documents we’ve been talking about for two years: his tax returns. If Trump does have a clear connection to Russians — if he owes them money, or if he has business partnerships with Putin allies — the returns may provide useful clues and would certainly be a worthwhile place to look.
Citation
George K. Yin, It Would Take Exactly One Senator to Get Trump’s Taxes, Politico (August 24, 2018).