Let’s start with the text of the Constitution, which in article one, section two gives the House the sole power of impeachment when the President commits high crimes and misdemeanors. We exercised that power on January 13th. The President, it is undisputed, committed his offense while he was president. And it is undisputed that we impeached him while he was president. There can be no doubt that this is a valid and legitimate impeachment, and there can be no doubt that the Senate has the power to try this impeachment.
So, the interpretation of the constitution on this point is that the act had to occur when the person was President. The impeachment occurred while they were President. So, the term "President" means anybody who is or has been "President".
But, the Constitution also says:
When the President of the United States is tried, the Chief Justice shall preside.
Now, the word "President" has a very specific meaning- only the current sitting President. According to comments here:
Senate Majority Leader Chuck Schumer said in an interview on MSNBC's "The Rachel Maddow Show" on Monday that "it was up to John Roberts whether he wanted to preside with a president who's no longer sitting. ... And he doesn't want to do it."
Aren't these two interpretations contradictory?