In the case that it were conclusively established that Trump engaged in electoral fraud, or engaged in corruption by coordinating illegally with the Russians, Trump would be impeachable. But Pence, who would not have committed an offense himself, may not be impeachable, as Michael J. Glennon hypothesizes in the Washington Post.
Glennon says that one view is that Pence would be impeachable. This is because if Trump had not engaged in electoral fraud and corruption, Pence would not have been elected because Trump would not have been elected.
Because of an unintended consequence of the 12th Amendment*,* ratified in 1804, Pence would be the first in the line of succession to replace Trump as President.
Before the ratification of the 12th Amendment, the Constitution allowed a president’s removal in the case of their electoral fraud, before they took office or after. Clearly, the Framers wanted the theft of an election through fraud and corruption to be an impeachable offense.
George Mason said, “Shall the man who has practiced corruption and by that means procured his appointment in the first instance, be suffered to escape punishment” by allowing him to stay in office?
Under the original Constitution, the impeachment of a president who gained his office through corruption "would not have resulted in a friendly takeover by a vice president of the same political party. When the impeachment clause was written in 1787, political parties did not exist." Initially, the system called for the nation to be led by the two most qualified individuals- the president and vice president, no matter their political party.
The system allowed this to happen by giving electoral college members two votes for the presidential office. The person who received the most votes would become president, and the runner-up would be vice president.
This system did not work once political parties emerged. In the election of 1796, the Federalist President John Adams was left with a Democratic-Republican vice president, Thomas Jefferson, who was also his rival.
The 12th Amendment was meant to remedy situations such as these. It requires electors to cast separate ballots for president and vice president, allowing candidates for president and vice president to run together on a party ticket.
This change did have important and unnoticed implications for impeachment. The election of two-people rather than one had the potential effect of allowing a vice president to benefit from the electoral fraud by the president, as long as the vice president did not commit an impeachable offense. Therefore, the same party would remain in power, benefitting from one person’s corruption.
There is, as Glennon writes, "every reason to believe that after the amendment’s adoption, the Constitution has continued to mean what it did in 1787: that the presidency ought not to be occupied by someone who attains it as the result of a stolen election."
Therefore, if Trump were impeachable, Pence would be impeachable too.