Almost a year in advance of the June 2020 primary, more than a dozen Democrats in New York have declared their plans to run, forming one of the most contentious congressional fields in the country at this stage. They are targeting some of the country’s longest-serving or most powerful politicians — most as first-time or outsider candidates, and some in the same district.

The phenomenon is not unique: Progressives across the country are plotting primary battles, spurred on by the victories last year of figures such as Representative Alexandria Ocasio-Cortez, as well as growing disenchantment with the Democratic Party’s old-guard wing. Early challengers have emerged in blue states including New Jersey and California…

“A.O.C. taught them that anything is possible,” Steve Israel, a retired congressman from Long Island, said of the challengers. “But theory runs into reality at a certain point in politics. And the reality is that there’s not a single Democratic incumbent who’s going to be caught unawares.”

One potential make-or-break factor is the involvement of Ms. Ocasio-Cortez herself, whose endorsement is highly coveted among progressive candidates. A spokesman for her declined to say whether she would get involved in congressional primaries.