It is now clear to effectively everybody except those blindly following Donald Trump that the Russians are actively tampering with the U.S. election. It may have started largely by accident, when the transparently bad "Romanian" Guccifer 2 persona deliberately muddled attribution of the DNC hack, but the subsequent disclosures and resulting disruption have turned this bit of improvisation into a significant strategy: sow chaos.
Making the problem worse, the United States is now distinctly vulnerable to chaos. With the rise of a dangerous post-fact demagogue in the form of Donald Trump and a more general hyper-polarization of parties that equates "compromise" with "betrayal," any actor who wishes to cripple the United States for a generation only needs to destabilize the election. Given Trump's talk of a "rigged" election, his defeat will already seem suspicious by his followers. Add in actual voting disruptions and the results may be catastrophic.
I won't discuss how a hostile actor could disrupt the voting process, simply because there are so many possible methods. Gather two or three security experts around the table and you will end up with a pile of horrors. Add both a case of beer and an expert in dirty tricks to the mix, and the scenarios will ensure that you will never sleep again.
Although there has already been a lot of focus on preventing specific methods, we also need to focus on more general plans for handling post-election chaos. A hostile attacker might launch disruptive attacks resulting in closed polling places, corrupted vote counts, or disrupted voter rolls.
Preparation begins with state and local governments. Local officials need to have plans in place for extending voting past the deadline at both single precincts or across entire counties. They should also ensure that there are large stocks of provisional ballots in case of disrupted rolls. These plans should factor in the possibility of corrupted votes where there aren't proper audit trails, perhaps even necessitating a re-vote. Officials should focus not just on "security" through planning to prevent particular scenarios, but also on resiliency, so that no matter what happens there is an ability to recover.
But preparation also requires a private commitment from Republican leaders. The party needs to quietly plan on how to respond should Trump lose. It seems highly likely that in a maliciously disrupted but still definitive election, Trump will refuse to concede and instead actively incite his base. The Republican party has no hope of preventing their candidate from doing destructive things, but can at least attempt to mitigate the damage by presenting a unified front against the worst Trump might offer on November 9th.