The Constitution should’ve placed adjudication of impeachment in the hands of the Supreme Court. The rationale for lifetime appointment to the court was to place the justices beyond political concerns. Not hard to see how politics would interfere with Senate impeachment.

Note, for instance, that no one has ever been impeached by the majority of their own party or anything close to it. It has always been a mostly party-line affair. (Andrew Johnson was Lincoln’s VP but he wasn’t a Republican.)

10 Republicans voted to acquit Clinton on one count, 10 Republicans joined the Democrats in impeaching Trump in January. Nixon would’ve been impeached (but Dems controlled both houses of Congress at the time.) The 44 GOP senators who voted against this trial raise a point:

We effectively have no ultimate check on a rogue presidency. We have a heavily partisan, contingent process that says if you do wrong while your party is less than 1/3rd of the senate you may run into problems.

Final point: it’s also worth remembering that the structure of impeachment was created before there were any political parties in the United States. The Framers couldn’t have predicted the extent to which partisan loyalty would corrupt and neuter the impeachment process.