WASHINGTON (AP) — Somewhere out there, the next Alexandria Ocasio-Cortez lurks. So wary House Democrats are amassing campaign war chests to scare off progressive upstarts from challenging them in primaries — or trounce them if they try.

A look at 41 incumbent House Democrats who face potential 2020 party primary opponents shows 16 have already stockpiled over $1 million in campaign funds. The figures from Federal Election Commission reports for the first six months of this year show that 20 raised over $500,000 during that period alone.

That’s not stopping challengers from targeting powerful committee chairmen and other well-financed incumbents, though the hurdles they face are clear.

So far only four Democratic challengers in these races have at least $100,000 socked away. The most is $352,000 by business consultant Marie Newman, who’s waging a primary rematch against Illinois Rep. Dan Lipinski, one of Congress’ most conservative Democrats. He has double her cash on hand, though she’s out-raised him so far this year.

“If you don’t have the money to fight an air war, you fight a ground war,” Monica Klein, a New York consultant who works with progressive Democrats, said of challengers who often lack money for TV commercials. “You try to out-organize your opponent and have those conversations at the doors, on the phone, face to face.”

Many challengers have barely started their campaigns but the early figures underscore a cold reality. Even with today’s energized and increasingly well organized progressive movement , incumbents’ fund-raising advantages — plus their name recognition and connections — are usually insurmountable.

The list of Democratic incumbents facing primary challenges will grow considerably, but most of those races won’t be truly competitive.

Ocasio-Cortez rocketed to influence and celebrity and is now a New York congresswoman after unexpectedly toppling 10-term veteran Rep. Joseph Crowley in their 2018 Democratic primary. Crowley, who was seen as potentially the next House speaker, spent over $3 million, multiples of Ocasio-Cortez’s expenditures.

A political unknown, Ocasio-Cortez relied on contributions of $200 or less for two-thirds of her money. Accumulating numerous small donations has become the gold standard of progressive campaigns, since givers can make repeated contributions and become campaign volunteers. So far this year, no major challengers in the races studied have raised AOC-like proportions of small donations.

Upsets like Ocasio-Cortez’s are rare. She, Rep. Ayanna Pressley, D-Mass., and two Republicans were the only primary challengers to oust any of the 376 House incumbents seeking re-election last year, meaning 99% of incumbents were re-nominated. Since World War II no more than 5% of incumbents have lost primaries, which happened in 1992.

Even so, leading Democrats are urging lawmakers to be aggressive fund raisers.

“My advice to any incumbent in this volatile environment: Take nothing for granted,” said former Rep. Steve Israel, D-N.Y., who once led House Democrat’s campaign organization.