A party official said lawmakers had chipped in $1.2 million for the House campaign committee after the appeal.
Among top Democrats, optimism has soared since Labor Day. Mr. Trump has handed them fodder via his Twitter provocations, and reports of deep internal divisions in his administration have added to a sense of a chaotic presidency — hijacking the news cycle.
Party leaders have closely tracked their leads in several public polls: During a meeting of congressional Democratic leaders on Wednesday evening, a top aide to Ms. Pelosi walked the group through a list of five recent polls that found voters nationally favoring Democratic congressional candidates over Republicans by double-digit margins.
Officials with the main House Democratic super PAC, the House Majority Fund, said their polling in August showed 17 incumbent Republicans trailing and six tied — nearly enough to recapture the majority without even factoring in the open seats the G.O.P. is defending. Strikingly, when the group this month surveyed some of the same districts where Republicans had unleashed a barrage of negative ads, it found that Democratic candidates had slipped only a little and that the races remained within the polling margin of error.
In the Senate, a mood of highly guarded hopefulness has spread among Democrats, who see a path to a majority that runs through a mix of right-leaning and solidly conservative states. By this point in the cycle, some in the party had feared that several incumbents would be headed to certain defeat, and once-inviting takeover opportunities would have slipped off the map, including in Tennessee and Texas. But both of those states remain competitive and a group of rust belt Senate Democrats, like Sherrod Brown of Ohio, seem secure.
“Despite the difficulty of the map’s geography, if there’s a big wave I think our odds are very, very good,” Senator Chuck Schumer, the Democratic leader, said in an interview, adding that when “you’re feeling the wave in September it rarely changes much by November.”
And the main reason Democrats are sensing a wave is obvious to party veterans.
“He won’t allow himself to get credit for the economy,” said James Carville, the Democratic strategist, referring to President Trump. Mr. Carville, who fashioned Bill Clinton’s “It’s the economy, stupid” mantra in 1992, continued: “He’s made himself bigger than the economy. Every conversation starts and ends with Trump.”