The question can be argued either way, but the better view is that Pence, too, would be impeachable. The reason is that, had Trump not engaged in electoral fraud and corruption, Pence, like Trump, would not have been elected. That Pence would still be first in the line of succession to replace Trump is the result of an unintended consequence of the 12th Amendment, which was ratified in 1804. The fate of the Republic ought not turn on a constitutional oversight.
Before the amendment’s ratification, the original Constitution permitted removal of a president for electoral fraud. It didn’t matter whether that misconduct occurred before or after a president took office. The framers clearly intended that theft of an election through fraud and corruption constitute an impeachable offense. James Madison was not concerned only about post-election perfidy when he worried, at the Constitutional Convention in 1787, that a president might “betray his trust to foreign powers.” George Mason seemingly spoke for many the same day when he asked, “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?
Removing a president who had procured office through corruption would not, under the original Constitution, 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. Madison and Jefferson’s Democratic-Republican party began to take shape only after the Constitution was drafted, with their famous 1791 “botany expedition” to New York. The initial system was designed to select as president and vice president the two individuals most qualified to lead the nation, whatever their political philosophy. It did this by permitting members of the electoral college to cast two votes for the office of president. The individual who received the most votes would be president, and the runner-up, vice president.
The system didn’t work as expected. With the emergence of political parties, the election of 1796 left Federalist President John Adams with a Democratic-Republican vice president, his archrival Thomas Jefferson. The election of 1800 created further problems, sticking Jefferson with the conniving Aaron Burr as vice president (whom Jefferson later ordered to be arrested for treason) — and only after a tie between the two in the electoral college led to a wrenching struggle in the House of Representatives that ultimately chose Jefferson.
The 12th Amendment was intended to remedy these ills. It did this by requiring electors to cast separate ballots for president and vice president. This enabled candidates for president and vice president to run together on a party ticket without competing with each other. Yet the change had critically important — and unnoticed — implications for impeachment. The election of a two-person ticket, rather than an individual, had the potential effect of permitting a vice president and his political party to benefit from electoral fraud by the presidential candidate so long as the vice president himself avoided committing an impeachable offense. A party’s ill-gotten gains — the presidency and all its appointments and prerogatives — would then remain in its hands even though its leader, the president, had been impeached and removed from office. Electoral corruption would still be rewarded.
That was not the amendment’s intent. Its object was to preclude the possibility of electing a president and vice president from different parties and to lessen the likelihood of an electoral college deadlock. It was not aimed at scaling back the availability of impeachment as a means of redressing electoral fraud. Nothing in the amendment’s ratification history indicates any intent to give a political party a continuing grip on the presidency should a president gain office, and be removed, because of electoral corruption. There is, to the contrary, every reason to believe that after the amendment’s adoption, the Constitution has in this respect continued to mean what it did in 1787: that the presidency ought not be occupied by someone who attains it as the result of a stolen election.
If Trump were impeachable for electoral fraud, therefore, Pence would be impeachable as well.