Here's why Republicans are in deep trouble in 2018

(CNN)There's a wave building in the country -- one that looks likely to threaten the Republican House majority come November 2018.

The signs are everywhere you look.
  • President Trump is at 37% approval nationwide
  • Democrats have a massive edge in the generic ballot, including a 14-point lead in the new CNN national poll
  • Deep divisions exist within the GOP between the Trump wing and the establishment wing
Political prognosticators are starting to take notice.
    On Monday, the Cook Political Report, a non-partisan handicapping tip sheet, moved 11 Republican-held seats into more vulnerable territory, a reflection, wrote House editor David Wasserman, of the quality Democratic candidates drawn to run by what looks like a very favorable national political environment.
    "Based on recent developments in races and conversations with candidates and operatives on both sides of the aisle, many races have the potential to become more competitive," Wasserman concluded.
    (Sidebar: For all the talk about how the political environment today doesn't dictate what that environment will be come November 2018, it does have a major impact. Today's political conditions help/hinder recruitment; good candidates win, bad ones don't.)
    Inside Elections, another political tip sheet, run by Nathan Gonzales, lists 48 GOP held seats and just 14 Democratic seats in its competitive House race rankings.
    The political models -- an attempt to forecast what the outcome of the 2018 House elections will be based on a series of objective factors that have proved influential in past elections -- are also beginning to point to increased odds of Democrats picking up the 23 seats they need to retake the majority.
    The Decision Desk HQ House model now gives Democrats a 46% chance of winning the majority, up from a less than one-in-three chance earlier this fall. (Some of that change, it's worth noting, is due to a change in the model; read more about that here.)
    Both the political prognosticators and the modelers are driven by the underlying numbers that historically have predicted which party will win elections -- particularly midterm elections. If those numbers change, the models shift and the political handicappers often adjust their predictions too.
    Which is of course possible! It's only October 17, after all.
    That said, the underlying numbers are daunting for Republicans.
    The generic ballot test -- if the election was held today, would you vote for the Democrat or the Republican for Congress? -- is generally regarded as a sort of weather vane to judge how strongly the wind is blowing and toward which party.
    And at the moment, the winds have shifted in Democrats' favor. Among all adults in the new CNN poll, 51% say they would vote for a Democratic candidate while 37% say they would choose a Republican one. Among registered voters, a narrower sample of those more likely to vote, the margin is even larger: 54% Democratic, 36% Republican.
    Democrats tend to have a three-to-five point edge on the generic ballot question. And yes it, is early. But such a wide margin in the generic ballot looks similar to October 2005 when Democrats had an eight-point edge; the party went on to pick up 30 seats and capture the majority from Republicans. It's also worth noting that Democrats' generic ballot edge in the CNN poll has never been below nine points since Trump became President.
    Then there is presidential approval. As Gallup has documented, midterm elections are always difficult for the president's party but especially so when the president is under 50% approval. Since 1946, the president's party loses an average of 14 House seats when the president is over 50%; the party loses an average of 36 seats when the president is below 50%.
    Trump's approval in both the CNN survey and the latest Gallup daily tracking poll is 37%. Trump's approval in Gallup data has never been at 50% or higher during his time in office. The closest Trump came was 46% in the January 24 Gallup tracker.
    Those numbers paint a dire picture for Republicans -- particularly when you consider what a Democratically-controlled House might mean for Trump's White House.
    To be clear: The numbers can change. And the polarization in the country when combined with the GOP-led congressional redistricting process after the 2010 census makes it tough for Democrats to make major gains without winning comfortably Republican seats.
    Waves -- especially big ones -- can wipe out places that were once considered impregnable. There looks to be a major wave building out in the political ocean right now. It could fizzle out before it reaches shore. But make no mistake: It's coming this way.