For Trump, the survey had damning news: Only 36 percent of respondents viewed his performance positively, and only 42 percent characterized him as “fit to serve as president,” while 56 percent did not.

The last first-term president to go into midterm elections with this kind of unpopularity was Harry Truman, whose party proceeded to surrender 55 seats in the House and 12 in the Senate in 1946. And even first-term presidents in much better shape than Truman and Trump usually watch their party suffer House and Senate losses. That’s the way the pendulum likes to swing.

The House is the better bet in 2018 for Democrats, whose excitement is intensified by some extraordinary numbers. The Brookings Institution noted that by the end of June, 209 Democrats not currently in Congress had registered with the Federal Election Commission to run. That was nearly triple how many Republican challengers had registered at this point in 2009, when the G.O.P. was galvanized by antipathy toward President Obama and new candidates were coming out in what was then considered droves. Republicans picked up 63 seats in the House the following year.

Democratic leaders aren’t talking about a bonanza like that. But maybe half that number? They point out that 23 Republican incumbents represent congressional districts that Clinton won last November. With a forceful swing of the pendulum, those seats could be the baseline of a bigger tally of red-to-blue triumphs.

“You have to shoot for the stars,” the Democratic operative Hilary Rosen told me. “You might just reach the moon.”

But even as Rosen said that, she hedged any prophecy of a rout, in a manner that spoke to the difficulty of properly calibrating optimism in 2018. She worried about Democrats’ policy agenda. She worried about the party’s tone. “I still think we lack a sunny, aspirational outlook,” she said. “We’re going down in the mud with Donald Trump.”

She added that the party wasn’t focused on change in the right, compelling fashion. “The change that Donald Trump was selling was blowing up the system,” she observed. “What’s our change? Is our change to patch up the system? Not very sexy.”