CONYERS, Ga. — Last November, election officials in a small Rhode Island town were immediately suspicious when results showed 99 percent of voters had turned down a noncontroversial measure about septic systems.

It turned out that an oval on the electronic ballot was misaligned ever so slightly and had thrown off the tally. The measure actually had passed by a comfortable margin.

The scary part: The outcome might never have raised suspicion had the results not been so lopsided.

Amid evidence that Russian hackers may have tried to meddle with last year’s presidential election, the incident illustrates a central concern among voting experts – the huge security challenge posed by the nation’s 10,000 voting jurisdictions.

While the decentralized nature of U.S. elections is a buffer against large-scale interstate manipulation on a level that could sway a presidential race, it also presents a multitude of opportunities for someone bent on mischief.

With an election year on the horizon, the Homeland Security Department has been working with states and counties to shore up their election systems against tampering.

“Always, there’s been a hypothetical. But clearly, now it is a real threat,” said Noah Praetz, election director for Cook County, Illinois. “The fact that we now have to defend against nation-state actors – Russia, China, Iran. It’s a very different ballgame now.”

Last year, Homeland Security disclosed that 21 states’ election systems had been targeted by Russian hackers. There was no evidence they actually penetrated the systems. Experts likened the activity to a burglar jiggling a doorknob to see if it is locked.

In the U.S. – from presidential races down to school board contests – elections are run to a very large degree by local governments, usually counties.

Small counties are less likely than the larger ones to have cybersecurity expertise and the latest technology.

“The proverb that a chain is only as strong as its weakest link is certainly applicable to our efforts to secure elections,” Brian Hancock, director of the testing and certification division for the U.S. Election Assistance Administration, said in a blog on his agency’s website.

After the “hanging chad” debacle in Florida threw the 2000 presidential election into confusion, Congress designated $3 billion to help states modernize their election systems.

But those machines are now more than 10 years old. A 2015 study by New York University Law School found that more than 40 states were using machines that were no longer being manufactured, and some election officials had to go onto eBay to find parts.