NEW YORK — Snapchat has no proven track record in election advertising. It shares only limited data about its audience, and caters mainly to a demographic that hasn't traditionally turned out in big numbers to vote. That's not stopping political campaigns from devoting more ad dollars to reach the millions of millennials who spend hours every week on the social-media app.

Democratic presidential candidate Hillary Clinton and her Republican rival Donald Trump have both stepped up their spending on Snapchat in the past month, according to a person familiar with the matter. The Clinton campaign is buying video ads with a targeted number of views by Snapchat's audience, while Trump is running an interactive ad seeking users' email addresses, said the person, who asked not to be named because the details aren't public. Both campaigns have used Snapchat's geographic photo-filter tools in the past at political rallies, but this is their first significant national advertising spend on the platform.

Though it started as an app for sending silly disappearing selfies, Snapchat has evolved into a legitimate news platform, now counting more than 150 million daily users, some three-fourths of whom fall into the coveted 18-31 age-group demographic. According to Nielsen Ratings, on any given day Snapchat reaches 41 percent of 18- to 34-year-olds in the U.S. More millennials watched the election's early debates on Snapchat than on TV. And some metrics indicate its users are more engaged. A Public Opinion Strategies study released in November said two-thirds of millennials who use Snapchat are likely to vote in the 2016 election, compared with 61 percent of the population overall.

"Snapchat has reached a point here in the U.S. where we can't ignore it anymore, and it's definitely earned a line on every campaign budget," said Tim Cameron, digital director of the National Republican Senatorial Committee. "We're really using demographics to reach a larger audience. It's not as specific as other tools but it works." The NRSC says it now spends more than 30 percent of its ad budget on digital messages, which include Snapchat.

Over the past few years, the startup started creating what it calls Live Stories, which get tens of millions of views by curating user-submitted photos and video from events, like the Super Bowl and the Republican National Convention. Snapchat also has deals with media companies like CNN and The Wall Street Journal to create mobile magazines for the app's Discover news section.

In May 2015, Los Angeles-based Snapchat introduced its political ad program, and quickly made several high-profile hires: Rob Saliterman, who previously led Google's political ad campaign, was brought on to run ad sales; and former CNN political reporter Peter Hamby joined to head news. While ad sales and news are kept separate at Snapchat, the two used their connections in the capital to bring Snapchat to Washington and promote it as a prime venue for political advertising. Snapchat has become the "MTV of Mobile," according to Zac Moffatt, who co-founded Targeted Victory, an ad agency that works with Republican candidates. "I would treat it like a cable network channel and say, I want to reach young people."

Just as MTV became a legitimate forum for political, youth-oriented debate in 1992 when, on one of its election-themed shows, an audience member queried another Clinton running for office whether he preferred "boxers or briefs," Snapchat is capitalizing on its young and engaged users to be taken seriously as a force in election advertising.

The first Democratic presidential candidate to take advantage of the platform for advertising was Sen. Bernie Sanders of Vermont, who ran a nine-day campaign on the site early in 2016 during the Iowa caucuses, after an appearance in a Live Story feature.

"We agreed to that because they showed us all the users that they have," said Hector Sigala, who ran social media for the campaign. "A lot of folks on Snapchat are voting. It was a staggeringly large amount."

As the election moved past the primaries, both the Clinton and Trump campaigns have paid for geofilters, images or text that appear over pictures captured within a limited geographic area, and Snap Ads, 10-second full-screen videos that play between other content.

"Most political professionals probably would not have predicted it even a year ago, but Snapchat has become a very effective strategy," said Dan Schnur, a professor at the University of Southern California and former political strategist. "Given the audiences' increased preferences for video rather than text-based (content) and given the increased competition, Snapchat is very well-positioned to inject the voter message into the voters' window of opportunity."