Princeton computer scientist and former White House Deputy CTO Ed Felten (previously) writes about the security lessons of the 2016 election: first, that other nation-states are more aggressive than generally supposed, and second, that you don't need to hack the vote-totals to effect devastation on an adversary -- it's sufficient to undermine the election's legitimacy by messing with voter rolls, "so there is uncertainty about whether the correct people were allowed to vote."
Attacks on legitimacy could take several forms. An attacker could disrupt the operation of the election, for example, by corrupting voter registration databases so there is uncertainty about whether the correct people were allowed to vote. They could interfere with post-election tallying processes, so that incorrect results were reported–an attack that might have the intended effect even if the results were eventually corrected. Or the attacker might fabricate evidence of an attack, and release the false evidence after the election.
Legitimacy attacks could be easier to carry out than election-stealing attacks, as well. For one thing, a legitimacy attacker will typically want the attack to be discovered, although they might want to avoid having the culprit identified. By contrast, an election-stealing attack must avoid detection in order to succeed. (If detected, it might function as a legitimacy attack.)
The good news is that steps like adopting auditable paper ballots and conducting routine post-election audits are useful against both election-stealing and legitimacy attacks. If we have strong evidence of voter intent, this will make election-stealing harder, and it will make falsified evidence of election-stealing less plausible. But attacks that aim to disrupt the election process may require different types of defenses.
Lessons of 2016 for U.S. Election Security
[Ed Felten/Freedom to Tinker]