Philadelphia, Pennsylvania - All summer, Pennsylvania's Democratic candidate for Senate, John Fetterman, had Facebook to himself.

While the Democrat’s social media machine rolled up memes, donations, and likes, his Republican rival for US Senate, Mehmet Oz, was dark on one of the world’s most popular platforms.

Both candidates were posting and sharing, but only one was spending on the kind of online advertising that has become standard in modern political campaigns.

From June through August, Fetterman ran dozens of Facebook ads, tweaked and targeted to dozens of specific audiences, that were seen millions of times. The flood helped Fetterman reach new voters, raise money, and get his message out.

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After outpacing Fetterman during their respective primaries, Oz stopped advertising entirely on Facebook for those three months, not picking back up until a handful of ads last month.

Fetterman spent more than $1 million on Facebook and Instagram from the primary through mid-September, according to an NYU Ad Observatory aggregation of data from Meta, Facebook’s parent company. That’s 38 times as much as Oz’s $27,000 in that period.

Nor was Oz spending advertising dollars elsewhere – when he didn’t have Facebook advertising, he wasn’t running TV or radio ads, either. The Fetterman campaign, flush with cash, went off-air for only a few weeks.

The early discrepancy helped Fetterman set the tone for their critical Senate race. Fetterman, the lieutenant governor, coasted to the Democratic primary victory and was raising significant money while Oz, the celebrity surgeon, endured a brutal and expensive GOP campaign and then had to slog through a recount.

"I would venture to bet that Fetterman’s engagement organically far surpasses any campaign in the country," said Ryan Rodgers, a Republican advertising strategist.

Fetterman has barreled through the election with a steady flow of small-dollar donations, a major portion of which comes from online advertising, which is where he has targeted the bulk of his Facebook spending.

Such fundraising can also be expensive: While Fetterman reported nearly $10 million raised in his latest campaign disclosure, he also spent nearly two-thirds of that in the same period.

"We have a lot of excited, grassroots supporters and because of that we’re able to get a lot of small dollar donors through Facebook," said Fetterman spokesperson Joe Calvello. "It really works for us because we have people who are excited about John and excited to click an ad or whatever it is and donate a few bucks."

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The campaign’s investment this summer, he added, helped them establish a foundation of donors to help now, when Oz and his allies are spending more on television.

But Oz had far less in the bank after a bruising and astoundingly expensive Republican primary and recount.

By June 30, about four weeks after the recount and in the midst of the period when Oz was dark on Facebook, his campaign fund was down to about $1.1 million left. Fetterman had nearly five times that.

"The only reason you pull back on digital or digital fundraising is because it’s not working, or you have no money," said Rodgers. "It’s one of those two reasons."

From the end of the May primary to the end of August, Fetterman spent $1.8 million on Facebook and Google ads, according to AdImpact, which tracks political advertising. Oz spent about $166,000.

After Fetterman’s free run for much of the summer, other Republican groups eventually stepped in to help narrow the gap, most prominently the Senate Leadership Fund, the super PAC affiliated with the Senate’s top Republican, Mitch McConnell. The GOP and Oz even outspent Fetterman early this month, according to AdImpact.

But overall, from June 1 to mid-September, Fetterman and his Democratic allies spent $7.3 million on ads for Facebook, Google, and streaming TV services. Oz and fellow Republicans spent $4.8 million, according to data from Bully Pulpit.

Of course, digital advertising is only one piece of the picture. Despite the vast gap online, recent polls suggest Oz has narrowed Fetterman’s early lead, and he and his Republican allies are now outspending Fetterman and his Democratic supporters on television.

While Oz went dark in advertising, he was still active on the campaign trail over the summer, holding dozens of events across the state.

"The more John Fetterman spends on Facebook, the closer the race gets," said Brittany Yanick, an Oz spokesperson. "That’s because races aren’t won on Twitter, they’re won on the issues – and John Fetterman is on the wrong side of all of them."

But Fetterman’s online tactics drew attention, keeping him in the news even as he spent the summer off the campaign trail while recovering from a May stroke. He’s well-suited to it, with a long-established following and a personal brand that resonates on social media.

Oz has a broad following, too, but one that’s likely more in tune to his former daytime TV show than his politics.

"If you don’t have a well-built audience, then building it is very expensive," said Rodgers, whose firm has worked on Pennsylvania Senate races. "(Fetterman) has the benefit of genuine, organic engagement… He’s done a phenomenal job of leaning into his brand, owning it and being really consistent."

Mike Schneider, a partner at Bully Pulpit, pointed, for example, to how Fetterman rode Oz’s "crudité" moment with ads that are "authentic and that stand out" and are "responsive" to what’s in the news.

"It hits this right mix of sort of eye-catching, funny, shareable and substantive, that I think we haven’t seen in a race in some time," he said.