"They may have vast resources, and in particular they may have more resources than the defenders—but they're still limited. Why? They'll throw enough resources at the problem to solve it, i.e., to hack the election, and use anything left over for the next problem, e.g., hacking the Brexit II referendum… There's always another target."
Too: there may be multiple parties attempting hacks. Getting your opponent to overextend (a war of attrition) is a time-honoured strategy.