SAN FRANCISCO — The campaign tool getting the most buzz in this year's midterm elections isn’t a Facebook ad or a viral YouTube video. Instead, it’s a technology nearly three decades old: the text message.

Political campaigns and parties say they’re sending many more texts this year than in past elections as they search for alternatives to social media. Those efforts have been boosted by newly popular software that allows volunteers to send thousands of texts an hour without violating federal rules about bulk texting.

With an assist from Silicon Valley, texting is beginning to supplant phone-banking and door-knocking as the most efficient way to talk to some voters, especially younger people addicted to their phones or those in rural areas who could be hard to reach in person.

“Many people much prefer texting to calls, and it makes sense,” said Roddy Lindsay, a former Facebook engineer who’s now co-founder and CEO of Hustle, a texting-software startup in San Francisco. The firm’s clients include the Democratic National Committee as well as nonpolitical clients, although only political clients have the freedom to send unsolicited texts under federal rules.

“When you get a call, it’s interruptive. You have to stop what you’re doing and take the call. But a text message, you can reply back at a moment of your convenience,” he said.

Several campaigns, including Democrat Andrew Gillum’s run for Florida governor, have credited texts with helping them pull off surprising victories this year. The Gillum campaign sent more than 1.5 million text messages to more than 750,000 voters, according to Hustle.

In Alabama, Democrats say they texted nearly every black voter in the state before a special election for U.S. Senate last December, which was won by the Democrat, Doug Jones.

But the proliferation of texting has also prompted a debate about whether regulation has kept pace with potential spam. Critics worry that the technology could be used for nefarious purposes, such as voter suppression or disinformation. In one example of the potential dark use of texting, some voters in Tennessee in July received anonymous text messages attacking two candidates in the Republican primary for governor.

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Federal rules prohibit unsolicited robo-texting, so Hustle and competitors such as Relay and RumbleUp have created software that makes sending huge numbers of texts relatively easy while still requiring a human to press the “send” button for each one.

The software allows political campaigns to upload lists with names, phone numbers and other personal information. Campaign workers create generic messages or personalized ones, sending them to friends or to strangers across the country — generally screened based on their political leanings.

Linda Hill, a volunteer for Build the Wave, a political group trying to turn out Democratic voters, said she sends texts on her iPad Mini from her sofa at home in Red Bank, New Jersey, and can easily contact 200 people in an afternoon. Some don’t respond or are unkind, she said, but others are pleasantly surprised to find out a human, not a robot, is behind the texts.

“It breaks down a lot of barriers," Hill said. "People think, ‘Oh, this is a real-life person who’s donating their time.'"

Person-to-person texting software is a twist on an old technology. The first modern SMS text was sent in 1992. In 2008, Barack Obama’s first presidential campaign used mass text messages to announce the selection of Joe Biden as his Democratic running mate.