I've seen a lot of stories out there today and yesterday at venues along the lines of The Hill, Politico and the Daily Beast pushing just the opposite theme.  So I think this Brookings Institution study is an important read just to counter the group think on this. To be aware that Dems planning counterattacks as if the GOP has become the Trump party might be somewhat of a self-induced illusion.

Yet a different theory, the over-confidence thing:

Trump's "red wave" could sink GOP

By Jonathan Swan @ Axios.com, Aug. 19

The president's contempt for mainstream polling and the media may come back to haunt him in November. Several top Republican operatives working on the midterm elections told me Trump's fanciful "red wave" predictions could depress Republican turnout and, ironically, serve to make any blue wave even bigger.

What we're hearing: One of those strategists told me he's detecting something interesting — and concerning — from focus groups of Trump voters. "You have Trump-MAGA loyalists, and their friends on Fox, who have reached a point of not believing polls and media people telling them things are going wrong, that I believe is actually causing the Republicans problems," the strategist told me, granted anonymity in order to be candid.

• "We've seen it in focus groups, with Republican base voters, where you'll come up with a hypothetical that the Democrats win, and people are like, 'That's not going to happen, that's stupid.' ... They're like, 'Oh, to hell with this crap, we were told Trump wasn't going to win. It's bullshit.'"
• "I would rather Sean Hannity get on TV every night and go, 'Oh my God, Nancy Pelosi could be Speaker and they're going to impeach Trump. You better get out to vote.'"

Between the lines: Questionable outlier polls like Rasmussen that favor the president have lulled and reassured Trump’s base. The president tweets out the polls, his media mouthpieces echo them, and his voters feel pacified — and, several top strategists I've interviewed fear, less motivated to show up in November.

• Trump isn't helping. The president is flatly denying what most polls indicate — saying he sees not a blue wave in November, but a giant "red wave."
• Republican strategists want the base to panic so they'll show up to vote. But instead, Trump is breeding complacency.

Another of the country's top House operatives told me [....]