Campaigning on a rainy recent morning, Mondaire Jones had plenty of reasons to be optimistic about his effort to unseat Representative Mike Lawler, a first-term Republican in the Hudson Valley in New York.

Democrats like Mr. Jones outnumber Republicans here by more than 75,000. The party is newly energized by the nomination of Vice President Kamala Harris. And Mr. Jones, a former congressman, has stockpiled millions of dollars for a campaign centered on reminding voters that Mr. Lawler backs former President Donald J. Trump and the overturning of Roe vs. Wade.

“His support is largely based on an illusion of him,” Mr. Jones said, referring to Mr. Lawler’s reputation as a moderate. “Here in the Hudson Valley, you can expect Democrats to vote for Democrats.”

And yet, with just 68 days to go, Mr. Jones is struggling to translate those partisan advantages into an actual lead, setting off alarm bells in New York and Washington that the party is once again facing unusually stiff local headwinds with middle-of-the-road voters put off by the state’s leftward lurch.

Democrats have a parallel fight on their hands in what would otherwise be considered the friendly territory of the Antelope Valley in California. There, the Democrat George Whitesides, a former NASA chief of staff and chief executive at Virgin Galactic, is laboring to unseat Representative Mike Garcia, a former military pilot from the northern suburbs of Los Angeles who has coasted to re-election since he first won in 2020.

Call it Democrats’ blue state dilemma, one that has helped make New York and California, two coastal bastions far from the presidential battlegrounds, the unlikely heart of the fight for control of the House of Representatives as summer turns to fall.