When Michael Malbin started crunching Federal Elections Committee data this summer to see how the 2018 midterms were shaping up, the sheer number of Democratic challengers blew him away.

Back in June, there were already 209 Democratic candidates entering House races who had raised at least $5,000, compared to just 28 Republican candidates.

Americans, check out this dramatic chart #2018 pic.twitter.com/EH4bRWG2ow — EricaGrieder (@EricaGrieder) December 5, 2017

When Malbin, the executive director of the Campaign Finance Institute, checked the updated numbers in the fall, the number of Republicans had risen to 71. By then, the number of Democrats had ballooned to 391. Of that number, 210 Democrats had raised at least $50,000 so far, and 145 had raised at least $100,000.

“I was simply stunned,” Malbin said. “I looked, and I said these are amazing numbers.”

It’s still too early to tell if a Democratic wave will sweep the House of Representatives in 2018. But the sheer number of Democratic challengers entering House races shows the makings of a wave.

To put this in context, Malbin says that the number of Republican challengers entering House races at this time of year is normal. The real surprise for him came on the Democratic side, where the numbers indicate a kind of energy around midterm races that he hasn’t seen before. The big drivers seem to be the reaction to Trump and congressional Republicans as they push unpopular policies like their current tax overhaul bill.

Since 2003, there have never been this many challengers before the midterm elections. The highest number before that was 78 Republican challengers in 2009, the year before the Republican wave during the 2010 midterms, where the GOP picked up six Senate seats and 64 seats in the House.

“From the Democratic side, there’s nothing that looked like these numbers, including the 2010 wave,” Malbin said.

Democrats have energy on their side. Can they harness it in 2018?

Malbin cautions that this data alone isn’t the crystal ball predicting the Democrats can flip the House in 2018. But it shows that key ingredients for a wave are in place; not only is there intense energy around Democratic candidates, they are also organized, entering races and fundraising early.

“This says potentially strong candidates are willing to take the risk on the Democratic side,” Malbin said. “That’s important, that’s a precondition for a wave, it’s not a wave.”

In a “wave” election, a large number of Congressional seats flip from one party to another, allowing that party to claim a majority. Waves often happen in the House of Representatives; the last big one was the Republicans’ sweep of the 2010 midterms, replacing the Democrats’ 256-seat majority with a Republican majority.

But it’s not just how many candidates are running — it matters where Democrats are running, and how many are lining up to challenging a single Republican incumbent. For instance, Malbin found that in California, eight Democratic challengers have filed FEC reports to run against the Republican incumbent, Rep. Jeff Denham.

Three other Republican incumbents from California, Illinois, and New York are in similar situations, with seven Democratic challengers filing to run against each of them. Some Democrats may be just duplicating their efforts.

“The most these 29 challengers can do in the general election is defeat four incumbents,” Malbin wrote in his analysis.

Another good sign for Democrats was the success of the 2017 governor’s races in New Jersey and Virginia, and a large number of diverse, progressive candidates winning municipal elections around the country. Long hyper-focused on big races, the Democratic party is starting to turn outward to focus on state and local elections, and numerous progressive groups are doing so as well.

During elections this year, groups like the Working Families Party, Bernie Sanders’ group Our Revolution, Run for Something, Democracy For America, Mobilize America and Sister District all endorsed or assisted local candidates, offering help like field organizing and fundraising assistance. These groups either complemented or filled in for the Democratic Legislative Campaign Committee, the Democratic party’s arm focusing on state legislative races.

“There’s always been a part of the Democratic Party apparatus that does state legislative races, but more groups are either being created or shifting their focus to state-level politics,” said Carolyn Fiddler, a political editor and senior communications adviser at the Daily Kos who tracks local races across the country.

After Trump’s election in November, Fiddler (who used to work for the DLCC) said she started seeing progressive groups getting engaged with state and local races in a way they hadn’t before.

“They started coming out of the woodwork last winter,” Fiddler said. “I’ve worked in state politics a long time, I’ve never seen anything like this.”

Malbin said these early signs are promising, but he also notes that progressive groups are focusing on many different things right now, including protesting Republican policies like the GOP tax reform bill and immigration policy. There’s much that could change before the 2018 midterms.

“There are so many targets that activists seem to be aiming at,” he said. “If it’s converted into electoral activity ... that would be a big plus for Democrats.”