Please stop hyperventilating and take a deep breath. This is just politics, not a Constitutional crisis. There are three coequal branches. If Congress starts to abuse its subpoena power for overtly political ends against either of the other branches, it may find the other branches can decide not to honor its subpoenas. Certainly the Executive can go to court and try to get a Congressional subpoena quashed. Congress subpoenaed Attorney General Eric Holder and he ignored the subpoena. The country did not fall apart. The subpoena for Trump’s tax return is even more dubious. The Democrats are simply trying to score some political points with their base. They know Trump will take it to court and it will be beyond 2020 before it is actually decided.