New York’s new map doesn’t just set Democrats up to win more seats, it also eliminates competitive districts. In 2020, there were four districts where Mr. Biden and former President Donald J. Trump were within five percentage points. There are none in the new map. Even the reconfigured district that stretches from Republican-dominated Staten Island to Democratic neighborhoods in Brooklyn is now, at least on paper, friendly territory for Democrats.

Without that competition from outside the party, many politicians are beginning to see the biggest threat to their careers as coming from within.

“When I was a member of Congress, most members woke up concerned about a general election,” said former Representative Steve Israel of New York, who led the House Democrats’ campaign committee during the last redistricting cycle. “Now they wake up worried about a primary opponent.”

Mr. Israel, who left Congress in 2017 and now owns a bookstore on Long Island, recalled Republicans telling him they would like to vote for Democratic priorities like gun control but feared a backlash from their party’s base. House Democrats, Mr. Israel said, would like to address issues such as Social Security and Medicare reform, but understand that doing so would draw a robust primary challenge from the party’s left wing.