Representative Alexandria Ocasio-Cortez is one of the best-known progressive politicians in the country, and her endorsement is sought by candidates running for president. Her firebrand style of politics has helped her raise millions of dollars and accumulate 6.9 million followers on Twitter.

But Ms. Ocasio-Cortez will not be appearing on the Working Families Party ballot line in the June primary or in the November general election because she failed to collect the required number of signatures — 15.

The loss of the ballot line will make no discernible difference in her re-election bid; she is still heavily favored to win the Democratic primary and the general election.

But ever since Ms. Ocasio-Cortez became the youngest woman elected to Congress in 2018, her every move has been scrutinized by both sides of the political spectrum, especially those on the right, who seldom miss an opportunity to magnify any perceived flaw.

Ms. Ocasio-Cortez was found to have only 13 valid signatures after her petitions were challenged by lawyers for one of her opponents in the Democratic primary, Michelle Caruso-Cabrera, a former anchorwoman for CNBC.

Martin Connor, a lawyer for Ms. Caruso-Cabrera, said one of the petitioners submitted by Ms. Ocasio-Cortez was a registered Democrat.

“Whoever wins the Democratic Party primary wins this race, but this sends a message,” Mr. Connor said. “The so-called progressive Working Families Party can’t deliver for A.O.C.”

Justice Phillip Hom of State Supreme Court in Queens ordered the Board of Elections to remove Ms. Ocasio-Cortez’s name from the W.F.P. line, where she would have run unopposed.

The 15 required signatures were a result of an executive order signed by Gov. Andrew M. Cuomo as a result of the coronavirus pandemic; the governor reduced the number of petitions required to make the ballot for political contests this year.

To secure a spot on the Working Families Party line, Ms. Ocasio-Cortez needed 14.58 signatures, which the Board of Elections rounded up to 15; Ms. Ocasio-Cortez’s campaign submitted 14, and one was knocked out.

Lauren Hitt, a spokeswoman for Ms. Ocasio-Cortez, said the campaign chose to stop collecting signatures because of the coronavirus pandemic.

“There’s no real impact on our race,” Ms. Hitt said, adding that Ms. Ocasio-Cortez still has the endorsement of the Working Families Party.

”We will work hard to ensure she wins her Democratic primary — where elections in the 14th District are always decided,” Sochie Nnaemeka, the New York state director of the party, said in a statement.

“As the pandemic was erupting, we did not believe it was appropriate to put canvassers or voters’ health at risk. We stopped collecting signatures — knowing it would not affect our ability to help in the Democratic primary,” she added.

The governor has placed New York in a state of shutdown to reduce coronavirus infections. More than 184,000 people in New York City have been infected by the virus, and there are more than 20,000 suspected and confirmed deaths.

Mr. Connor said that the coronavirus pandemic should not have been an excuse. Candidates normally collect many more signatures than required to get on the ballot.

“The lesson is get your butt out there the first week and get the signatures,” Mr. Connor said.

Ms. Caruso-Cabrera also taunted Ms. Ocasio-Cortez, whose district covers parts of the Bronx and Queens. “The A.O.C. campaign is in shock,” she said in a statement.

Ms. Hitt rejected that assertion and said that the W.F.P. endorsement would still carry “a certain set of values” that voters appreciate.

“Congrats to them for finding technicalities, but it’s not going to change the outcome of the election,” Ms. Hitt said.