https://www.washingtonpost.com/blogs/plum-line/wp/2018/05/15/the-truth-is-about-to-catch-up-to-trump-he-has-giuliani-to-thank-for-it/

Charlie Cook (of Cook Political Report?), quoted in WaPo:

«Democrats have less margin for error. Things were going so well for them last fall that it was unlikely that primaries nominating exotic candidates would prevent them from winning a bare 218-seat majority. Now Democrats can ill-afford nominating too many candidates who are badly flawed or too liberal to win a district that has voted Republican in recent years.»