The 2018 House playing field continues to go downhill for Republicans

(CNN)The 2018 House playing field keeps shifting and expanding -- and all of the changes favor Democrats.

The Cook Political Report, a leading independent political handicapping site, moved 13 races in Democrats' favor on Friday morning -- changes that mean that 50 Republican-held seats are rated as competitive while just five(!) Democratic seats are seen that same way. That's 10 times as many! (That quick math should win me the Fields Medal, right?)
If you count up only the Republican seats that Cook rates as "toss-ups" (21) or leaning or likely to go to Democrats (eight), the minority party has six more seats than it needs to retake the majority. (Republicans have a 23-seat edge at the moment.)
"Overall, Democrats would need to win 27 of the 55 competitive races to win a majority," wrote Cook's David Wasserman. "We continue to view Democrats the slight favorites for House control."
    The Cook ratings are in keeping with other handicapping of the House playing field. CNN rates 43 Republican seats as competitive as compared to just six for Democrats. "Inside Elections," another non-partisan campaign tipsheet, puts 36 GOP seats and just eight Democratic-held ones in its most competitive category.
    The simple fact is this: As we get closer to the 2018 midterms -- only 214 days left! -- Republican vulnerabilities come into much clearer focus.
    Some of that is to be expected. After all, there have been only three midterm elections in the last 90 years -- 1934, 1998 and 2002 -- in which the party that controlled the White House didn't lose seats. (And in each of those elections there was a massive external event shaping the historical anomaly: The Great Depression, impeachment, and 9/11, in that order.)
    That reality is compounded by the galvanizing effect that President Donald Trump has had on the Democratic base. In election after election since Trump's 2016 win, Democrats have heavily overperformed Hillary Clinton's showing -- the result of a major enthusiasm gap between their base and the GOP one.
    While candidates like Sen. Doug Jones (D-Ala.) and Rep. Conor Lamb (D-Pa.) get most of the attention -- because they won -- CNN's Harry Enten has detailed how even in special election losses since 2016, the Democratic candidates have far outperformed Clinton.
    Because there are 200+ days until the midterms, there's still time for this to change, of course. One of the potential benefits to Republicans of Trump's aggressive approach to the border of late -- sending National Guard troops to monitor it until the wall is built -- may well be something of a rallying effect from the Democratic base.
    That could help Republicans close the enthusiasm gap somewhat. But it's hard to imagine anything making Democrats' less enthusiastic about turning out this fall to send a very clear message to President Trump.
    If turnout patterns in November are anything close to what they've been in 2017 and 2018 to date, Republicans could lose many more than 23 seats. The GOP faces considerable exposure and there's no oasis in sight.