New York battle lines:

• Rep. Joe Crowley in New York's 14th district has served in Congress since 1999. Alexandria Ocasio Cortez, a 28-year-old progressive activist, is challenging him.
• Rep. Yvette Clarke in the state's 9th district has held her Brooklyn seat since 2006. She is going against Adem Bunkeddeko, a 30-year-old ­son of Ugandan refugees, who said of his opponent: “I think her tenure has been a failure."
• Rep. Eliot Engel, who's served 30 years in Congress, is facing three challengers, and has spent $1.3 million against one them, Jonathan Lewis.
• Rep. Carolyn Maloney in the state's 12th district was first elected in 1992 and is fighting off a challenge from 34-year-old Suraj Patel, a hotel executive who says Maloney uses "political tactics from last century that excite no one and completely overlook the need to bring new people into the process.” (He has been criticized for making fake dating app profiles to lure voters.)

Democratic divide: Single-payer health care has become a litmus test for Democratic candidates running to unseat Rep. John Faso in the 19th district. One of the candidates, Brian Flynn, has an ad out arguing that they "may all be Democrats, but I support Medicare for all, and they don't." Since then, both of his challengers have moved to the left, endorsing a "move in the direction of a single-payer system."

Be smart: Even if these challengers don't win this cycle, they're building a bench of progressive candidates for 2020 and beyond.

Correction: An earlier version of this article said Flynn's challengers had endorsed single payer. It has been reflected to show they endorse an eventual path toward single payer.