The Democrats can request all they want to, and apparently that is going to be a lot. Trump is under no compulsion to comply, so what are they going to do to him if he does not?
Impeachment may be an option, but “High Crimes and Misdemeanors? An undefined and indefinite phrase at best, it would be a pretty hard case to make even with the most possessed of prosecutors.
They are not requesting the tax returns from President Trump.
The Chairman of the House Ways and Means Committee has the authority to request individual tax returns directly from the IRS. That's how they are finally going to get their hands on the detailed returns from Trump, his businesses, his childrens' returns, and on and on.
Expect them to be leaked, too. Expect them to become the basis for subpoenas against his businesses by the New York City prosecutors, too. Expect them to become weapons for Democrats to destroy Trump financially, destroy his brand, destroy his real estate empire, if they cannot destroy him politically.
-PJ