During a recent interview on CNN, Ted Cruz described what he considers an under-reported threat to national security: Electromagnetic pulse, or EMP, attacks. He explained that this danger involves "a nuclear weapon detonated in the atmosphere, that would take down our electrical grid," and warned that "it could kill tens of millions of Americans. And all Iran would have to do is fire one nuke into the atmosphere — they don't need to hit anything, they just need to get it above the Eastern seaboard, and they could kill tens of millions."

Senator Cruz had also mentioned EMP attacks during the New Hampshire GOP debate, warning that North Korea could attach a nuclear device to a satellite and set off an EMP that "could take down the entire electrical grid on the Eastern seaboard, potentially killing millions."

He wasn't alone in sounding the EMP alarm. When Ben Carson was still in the race, he warned of "enemies who are obtaining nuclear weapons that they can explode in our exo-atmosphere and destroy our electric grid." Jeb Bush called the threat of an EMP "scary as crap. It's one of the scariest things that could happen."

These candidates, past and present, were right to point out the threat from EMPs, but their claims overlook that an EMP event can be caused not only by a nuclear attack, but by a solar event as well. And although the nuclear variety is the much more frightening and sensational, it is also less predictable.

A nuclear EMP would involve a device exploding at high altitudes above the Earth's surface. The detonation would create a series of shocks that would threaten unprotected electronics (such as personal electronics and communications lines) and long-line network systems (including electric transmissions, communications lines, and both oil and natural gas pipelines). Such damage would be serious and deadly.

However, the nations capable of launching a nuclear EMP device that could seriously degrade our critical infrastructure are unlikely to, either because they are our allies (e.g., France, Israel, and the United Kingdom) or because they have too much to lose from retaliation (e.g., China). Rogue states with nuclear weapons, like North Korea, lack the technology necessary to miniaturize the weapon, mount it on a missile, and fire it accurately enough to cause a widespread EMP catastrophe.

Finally, although it's possible for terrorist or criminal groups to acquire nuclear weapons, they lack the organization necessary to operate and detonate it, let alone the technological know-how to miniaturize and launch it 30,000 feet into the air.

The more likely source of an EMP event is the sun itself. Solar events are not merely theoretical: In the Solar Storm of 1859, or the Carrington Event, a coronal mass ejection emitted so much energy that it coupled with telegraph wires, causing forest fires, lighting paper on fire, and destroying the newly-laid intercontinental telegraph line that ran under the Atlantic Ocean.

That was over 150 years ago, when the world did not rely on electrical systems. In 2010, Lloyds of London and Atmospheric and Environmental Research published a report indicating that a Carrington Event-level solar EMP today would cause extended blackouts for 20-40 million Americans and cost, at a conservative estimate, $600 billion to $2.6 trillion. (By comparison, Hurricane Katrina caused approximately $125 billion in total economic losses.)

What, exactly, could a solar or nuclear EMP event threaten? For starters, the electric grid is vulnerable, which means that major industries are, too. Telecommunications are directly threatened by any damage to the electric grid. The event would also have indirect, but major, impacts on banking and finance, fuel and energy, transportation, water supply, and many other infrastructures and industries that are central to the everyday life of Americans. A solar EMP is, despite being caused by natural events, a major national security threat to the United States.

To protect against the damage that an EMP could inflict, the federal government needs to team up with the private sector. Much of the government's responsibility for mitigating EMP events involves national defense and intelligence operations, as candidates have suggested. But the federal government must also improve early warning systems for coronal mass ejections. It could also promote EMP protection and preparedness by ensuring that key infrastructure assets are identified and hardened to the extent practical by private sector owners and operators.

It's encouraging that several presidential candidates, including one who still has a chance of winning the Republican nomination, have recognized the dangers of EMP. If only they understood the true nature of the danger posed by a solar EMP event, and the steps required to protect against it.

Mr. Colangelo is executive director of Consumers' Research, the nation's oldest independent consumers' interest organization. Thinking of submitting an op-ed to the Washington Examiner? Be sure to read our guidelines on submissions.