Just how big is the Democratic wave going to be this fall?

key house race update cillizza orig bw_00003309
key house race update cillizza orig bw_00003309

    JUST WATCHED

    More House races that should worry the GOP

MUST WATCH

More House races that should worry the GOP 01:47

Washington (CNN)If you're wondering whether the 2018 midterm elections will be a Democratic wave, you're asking the wrong question. The fall contests will undoubtedly be a national referendum on Donald Trump's first two years in office and, if polling (and history) is to be believed, Democrats will reap the electoral rewards from the negative views of the President among a majority of the country.

The real question you should be asking yourself is how large will this wave be? Will it be small (15-20-seat Democratic pickup), medium (25-35-seat pickup) or large (35-plus-seat pickup)? (Reminder: Democrats need to net 23 seats to retake the House majority.)
Judging by what happened in an Arizona special election on Tuesday night, the "large" wave option now looks to be very much in play.
"If the only data point you had to go on was last night's #AZ08 result, you'd think a 30-40 seat Dem House gain in Nov. would be way low," tweeted David Wasserman, the House editor of the non-partisan Cook Political Report.
    Here's why.
    Yes, Republican Debbie Lesko beat Democrat Hiral Tipirneni in the 8th District special election triggered by former Republican Rep. Trent Franks resignation amid claims of sexual harassment.
    But, Lesko's margin -- 5.2 percentage points -- is far less than the 25 points Mitt Romney won the district by in 2012 or the 21-point margin for Trump in 2016.
    In a vacuum, that result could be dismissed as simply a one-off -- a special election with two little-known candidates yadda yadda yadda. The problem is that what happened in Arizona on Tuesday -- dramatic Democratic overperformance of the party's candidate -- is far from an isolated incident.
    Check this out:
    According to CNN's own resident big brain Harry Enten, "the average improvement for the Democrats has been 17 percentage points versus the partisan baseline. That's better than any party out of power has done in the lead-up to a midterm cycle since at least 1994."
    The lesson here is obvious: Driven by their distaste for Trump, the Democratic base is turning out in droves. The Republican base, fat and happy with control of the House, Senate and the White House, is less galvanized. And, loosely-affiliated partisans seem to be acting much more like Democrats than Republicans in the elections since Trump won the White House.
    The threat to Republicans should also be obvious: A month removed from Democratic Rep. Conor Lamb winning a southwestern Pennsylvania seat that Trump carried by 20 points in 2016, Lesko eked out a victory in a seat where Trump won by 21.
    If seats like Pennsylvania's 18th and Arizona's 8th are competitive, then the number of Republican vulnerabilities is bigger than anyone thought.
    "There are 147 GOP-held House seats less Republican than #AZ08," tweeted Wasserman. "It's time to start rethinking how many of those are truly safe in November." (A bit of quick math produces this: 62% of the 237 seats Republicans will hold -- once Lesko is sworn in -- are less friendly for the GOP than Arizona's 8th.)
    Tweeted The New York Times' Nate Cohn: "There are a bunch of open districts that aren't considered top-tier races by most analysts --TX-2, TX-6, FL-6, OH-16, TX-21, FL-15 -- that, by the numbers, look plausibly competitive in a wave election. On paper, they're better Dem targets than, say, AZ-8/KS-4/PA-18."
    If the Republican playing field is anywhere close to that 147 number, that makes it at least twice as large as most non-partisan political handicappers currently believe it to be.
    CNN rates 73 Republican seats as potentially competitive. The Cook Report puts 84 Republican districts in that competitive category while Inside Elections, another independent campaign tipsheet, has 58 competitive GOP seats.
    What we know is that in a wave, seats get washed away that many people didn't even know were competitive. The larger the number of vulnerable districts, the more that could potentially get washed away.
    And, from a more practical standpoint, Republicans and their aligned super PACs have only so much money to spend in 2018. The more of their own seats that come online as competitive, the more hard decisions party committees and super PACs will have to make about who gets money and who doesn't.
    Simply stated: Arizona's 8th District results suggests that the coming wave is big -- and getting bigger.
    CORRECTION: This story has been updated to reflect the number of House seats held by Republicans.