Recently, former president Trump turned down the opportunity to testify at his own impeachment hearing.
However, given the magnitude of the charges, could he be compelled to testify by subpoena? Or could Trump plead the fifth, thereby avoiding having to testify against himself?
The basis for this question is that there's a bit of debate upon whether impeachment itself is a legal process or a political one. The criteria by which a president is impeached is deliberately vague, but the penalties for a successful impeachment are partially legal and partially political (i.e. removed from office, barred from serving again; but this does not otherwise deprive the defendant of their liberty (though other legal charges may)). Regardless, because impeachment does not have any stated criteria other than, "Treason, bribery, or other high crimes and misdemeanors," those penalties could theoretically come about simply by virtue of Congress holding a 2/3 majority of an opposing party.
Given that the 5th amendment protects a defendant from being a witness against themselves in a criminal proceeding, could Trump exercise that protection to avoid being compelled to testify before Congress?