Alexandria Ocasio-Cortez, the 28-year-old socialist who secured a stunning Democratic primary win over New York Rep. Joseph Crowley last month, is getting slammed by some Democrats who say her fringe views are damaging the party.

Former Sen. Joe Lieberman, now a Democrat turned independent, wrote an op-ed for The Wall Street Journal on Tuesday calling on Democrats to vote for Mr. Crowley, who will appear on the ballot of the Working Families Party in November.

“On Election Day, his name will be on the ballot as the endorsed candidate of the Working Families Party,” he wrote. “But for Mr. Crowley to have a chance at getting re-elected, he will have to decide if he wants to remain an active candidate. I hope he does.”

Mr. Lieberman argued that Ms. Ocasio-Cortez’s policies “are so far from the mainstream, her election in November would make it harder for Congress to stop fighting and start fixing problems.

“Ms. Ocasio-Cortez is a proud member of the Democratic Socialists of America, whose platform, like hers, is more Socialist than Democratic,” he wrote.

“Her dreams of new federal spending would bankrupt the country or require very large tax increases, including on the working class,” he continued. “Her approach foresees government ownership of many private companies, which would decimate the economy and put millions out of work.”

In an interview published Tuesday by The Hill, Democratic Rep. Alcee Hastings warned Ms. Ocasio-Cortez that alienating moderates in the party would hurt her chances of getting anything done if she’s elected.

“Meteors fizz out,” he said. “What she will learn in this institution is that it’s glacial to begin with, and therefore no matter how far you rise, that’s just how far you will ultimately get your comeuppance.”

Without mentioning Ms. Ocasio-Cortez directly, Democratic Sen. Chris Coons last week slammed the party’s “relentless race to the left” and called on fellow Democrats to abandon radical ideas like “Abolish ICE” — a policy Ms. Ocasio-Cortez has endorsed.

“If we as a Democratic Party are going to move from a minority at every level that is dedicated to resistance, to a majority that is capable of governing, we have got to move from grievance to optimism,” Mr. Coons said. “And we’ve got to abandon a politics of anxiety that is characterized by wild-eyed proposals and instead deliver ideas and practical solutions.”

Last week, Ms. Ocasio-Cortez accused Mr. Crowley of still running against her in the general election.

.@repjoecrowley stated on live TV that he would absolutely support my candidacy.



Instead, he’s stood me up for all 3 scheduled concession calls.



Now, he’s mounting a 3rd party challenge against me and the Democratic Party- and against the will of @NYWFP.https://t.co/Xvb6Jk8N8q — Alexandria Ocasio-Cortez (@Ocasio2018) July 12, 2018

Mr. Crowley responded saying he’s not actively running in the general election.

Lots questions about WFP line. Was honored to have their support. I’m not running. For record you can only be removed from the ballot if 1) you move out of NY; 2) die; 3) be convicted of a crime; 4) accept a nomination for another office (in a place I don’t live). — Joe Crowley (@JoeCrowleyNY) July 12, 2018

Democratic Rep. Lacy Clay called Ms. Ocasio-Cortez “petty” for attacking Mr. Crowley publicly.

“Once an election is over and you win, why are you still angry?” he told The Hill. “I think it’s a lack of maturity on her part, and a lack of political acumen, for her to be that petty.

“We as Democrats better figure out who the real enemy is. And it’s not each other,” he added.

Democratic Rep. Bill Pascrell said Ms. Ocasio-Cortez is “no doubt” making enemies in the party.

“She’s carrying on and she ain’t gonna make friends that way,” he told The Hill. “Joe conceded, wished her well, said he would support her … so she doesn’t know what the hell she’s talking about.”