What are the chances that Democrats will take over Congress next year? Obviously, it’s early, and polling isn’t going to give us a very reliable picture just yet. But the number of candidates from each party getting in line to run can give us some useful hints about how things will shake out.

Last year, I wrote a few pieces about the numbers of candidates who had filed to run for Congress. Since 2010, there had been more Republicans than Democrats filing to run for Congress in every election cycle.

Ed Kilgore ran a similar analysis recently at New York Magazine, drawing from a longer time series made available by the Campaign Finance Institute. The main finding was that Democrats hold an enormous advantage in early candidate filings for the 2018 midterm elections. In particular, if we limit the analysis to the number of challengers to House incumbents who have filed for next year and have raised at least $5,000 — in an effort to narrow our sample to truly viable candidates — we see a record advantage for Democrats right now.

Who wants to run for the House? Number of House challengers who raised at least $5,000 by June 30 of year prior to election ELECTION CYCLE DEMOCRATS REPUBLICANS 2004 22 – 29 – 2006 48 – 24 – 2008 57 – 49 – 2010 40 – 78 – 2012 42 – 40 – 2014 45 – 52 – 2016 44 – 28 – 2018 209 – 28 – Source: Campaign Finance Institute

But what exactly does this mean? Yes, Democrats had twice the number of challengers that Republicans did in 2006 and then took over the House in that election, while a similar advantage yielded similar payoffs for Republicans in 2010. But should we necessarily expect an advantage in the number of early candidates to lead to election victories?

In the chart below, I have plotted the Democratic advantage in early House challengers against the number of House seats won by Democrats since 2004. As the chart suggests, while there is a pretty small number of data points, this is a very strong relationship. Each additional percentage advantage in early candidates yields about 2.5 additional House members in the election.

Why do we see such a strong relationship? It’s not precisely that the number of candidates causes a party to win more seats. After all, there are only so many House seats in play. What a large number of challengers does create is a better recruitment environment. If there are several challengers from whom to choose in a particular race, a party can pick the strongest nominee.

Political science research suggests that the recruitment of high-quality candidates explains a good deal of election outcomes — if a party can convince a large number of skilled and experienced candidates to run for office, those candidates tend to do better and the party tends to win more seats. Indeed, the recruitment of quality candidates helps explain the development of the incumbency advantage in 20th century American politics. Finding strong candidates was Newt Gingrich’s approach prior to the 1994 Republican landslide, just as it was Rahm Emanuel’s strategy for 2006.

Other factors will affect just how successful those recruitment efforts will be, of course. If a House member looks safe and the political fundamentals (including the state of the economy and the president’s popularity) don’t look like they’re going to make incumbents unpopular, it will be hard to convince, say, a state legislator in a safe district to jump into a difficult and expensive congressional race.

But the environment right now suggests that Republican incumbents are vulnerable. President Trump’s approval ratings are in the mid-30s, even amid a strong economy, and it’s hard to see how the environment will improve much for the GOP by next year. And one way Democrats have been responding to Trump’s various norm violations is by running for office.

Of the 237 House challengers who raised at least $5,000 for the 2018 midterms by the end of June, 209 of them (88 percent) are Democrats. If we were to plug that into the regression line above, it suggests Democrats would pick up 93 House seats. This figure seems highly improbable given the number of seats that are actually competitive, as Kilgore and Kyle Kondik note. But it does suggest strong potential gains for the Democrats next year.

Of course, it’s still early, and the people who went to rallies last January and said “Hell yes, I’m running for Congress!” might ask “What was I thinking?” by the time next January rolls around. And it’s hard to know just what the political system will look like by this time next year given the rapid pace of events lately. But indicators thus far suggest a strong year for the Democrats.