Hacking an election . . .

. . . is possible. Is it probable?

Yes.

The Washington Post:

Reports this week of Russian intrusions into U.S. election systems have startled many voters, but computer experts are not surprised. They have long warned that Americans vote in a way that's so insecure that hackers could change the outcome of races at the local, state and even national level.
Multibillion-dollar investments in better election technology after the troubled 2000 presidential election count prompted widespread abandonment of flawed paper-based systems, such as punch ballots. But the rush to embrace electronic voting technology - and leave old-fashioned paper tallies behind - created new sets of vulnerabilities that have taken years to fix.
"There are computers used in all points of the election process, and they can all be hacked," said Princeton computer scientist Andrew Appel, an expert in voting technologies. "So we should work at all points in that system to see how we make them trustworthy even if they do get hacked."

Story.