Impeachment Should Be Confined to Actions While in Office
Even Investigating Pre-Inauguration Events Could Undercut Process
Although there may be a temptation to include alleged hush money payments, and even alleged wrongdoing associated with Russia's participation in the 2016 election, precedent suggests that this extraordinary constitutional remedy should be used solely to deal with conduct once a candidate takes office.
The strongest precedent comes from the attempted impeachment of Vice President Schuyler Colfax.
Then the House Judiciary Committee concluded that impeachment "should only be applied to high crimes and misdemeanors committed while in office and which alone affect the officer in discharge of his duties as such, whatever may have been their effect upon him as a man, for impeachment touches the office only and qualifications for the office, and not the man himself."
Moreover, according to a recent analysis by the Congressional Research Service [CRS], "it does not appear that any President, Vice President, or other civil officer of the United States has been impeached by the House solely on the basis of conduct occurring before he began his tenure in the office held at the time of the impeachment investigation, although the House has, on occasion, investigated such allegations."
Indeed, the CRS also reported that Congress has identified three types of conduct that constitute grounds for impeachment, and they all require that the acts occur when the person is already in office. These categories, which may not be exhaustive, include:
(1) improperly exceeding or abusing the powers of the office;
(2) behavior incompatible with the function and purpose of the office; and
(3) misusing the office for an improper purpose or for personal gain.
In short, for impeachment, what is arguably necessary is some serious wrongdoing that counts, in Alexander Hamilton's words, as "the abuse or violation of some public trust," and not even more serious crimes and other wrongdoing which may have occurred prior to the time a person assumed office.
If this precedent is still seen to control and is followed, Trump could not legally be impeached, for anything he may have done before assuming the presidency.
Moreover, any attempt to include allegations related to pre-inauguration conduct would provide the President with to argue that the process is unconstitutional, even if there are many counts in the articles of impeachment relating to conduct such he assumed office.