The field of front-runners may grow larger before it grows smaller after two of them — former Vice President Joe Biden and Sen. Kamala Harris — failed to parry an avalanche of attacks during the Democratic Party’s second night of debates in Detroit, said The Post’s Night 2 debate expert Christina Greer.

Greer was impressed by former HUD Secretary Julián Castro and New Jersey US Sen. Cory Booker, both of whom she said made compelling cases to join the top-tier ranks during the Wednesday night forum at the Motor City’s historic Fox Theatre.

“It was a tie between Booker and Castro. I think both of them emerged with some real concrete points. They were clear and decisive, landed some definitive jabs and were concrete against the president,” she said. “I think their post-debate performance will give them a bump, I’m not sure how long the media will be interested in the two of them.”

That means the field of people Democratic voters devote attention to will likely grow larger, even as the number of pols running for the party’s presidential nomination begins to shrink.

“I think the front-runner race gets more crowded as some of the more tertiary candidates fall by the wayside,” the Fordham professor and veteran political commentator added.

Greer said there was one clear loser in tonight’s debate: New York City Mayor Bill de Blasio’s quixotic quest to go from Gracie Mansion to the White House.

“De Blasio was the loser tonight,” she added. “Not even the [Eric] Garner question was directed at him.”

WINNERS

Julián Castro

Analysis: Presented a strong case for attendance in the next debate with measured and thoughtful answers to complex issues — including on Eric Garner and President Trump’s racist tweets. He started to lay a foundation as to why he deserves to be in the conversation. It could be enough to catapult him into the conversation as a top-tier candidate.

Cory Booker

Analysis: He had a strong night on almost all issues and had quite a few moments that resonated with the audience. He even got in a few jabs on Biden and retweetable moments — especially his “Kool-Aid” exchange with the former vice president on Biden’s controversial 1990s crime bill.

LOSERS

Bill de Blasio

Analysis: Consistently tried to insert himself into debates and goad Biden into a fight — and was ignored. There’s nothing worse than that in a debate. Even the moderators swatted him away at times. His answers on Eric Garner and lead poisoning were insufficient. Who would be interested in his campaign beyond those doing business with New York City?

Michael Bennet

Analysis: He did not have any standout moments nor did he present any unique ideas. Who is his audience and who would be interested in seeing his campaign go further?

Tulsi Gabbard

Analysis: Anyone who needs to look at notes when speaking about the crisis on the border is problematic. She had Harris in her crosshairs and was successful in landing a punch on the former prosecutor’s record. Beyond that, she did not contribute much beyond talking points. Even as a military veteran, she did not have a strong grasp of foreign policy and I’m not sure if she makes the next debate.

TREADED WATER

Kirsten Gillibrand

Analysis: She didn’t land any strong punches but managed to poke some holes in Biden’s record on women’s rights. Her track record of winning in a GOP-friendly congressional district and appeals to women could sway suburban voters — giving her campaign a rationale to go forward.

Joe Biden

Analysis: He was on the defensive and sometimes failed to complete his thoughts. He couldn’t parry the furious attacks over his 1990s crime bill — and his habit of leaning on President Obama to avoid tough questions was exposed. He did not look like a front-runner tonight, though his status in the race will shield him from the fallout of yet another mediocre performance.

Kamala Harris

Analysis: She came in riding high on her past debate performance but struggled under constant attacks from the rest of the stage. Her defense of her health care plan was not strong and her record as California AG may likely come back to haunt her. Harris’ answers seemed quite technical and her proposals on prisons and federal support for abortion didn’t seem to connect concretely.

Andrew Yang

Analysis: Everything about his campaign is built around economics, which is how he answered every question Wednesday night. His lack of governmental experience may scare some voters but excite others who are fed up with DC politics. He is a necessary voice but still does not seem to be a viable presidential nominee.

Jay Inslee

Analysis: For those paying attention, Inslee consistently pointed to successful policy initiatives under his watch in Washington state. He moved beyond his major issue of climate change and presented clear answers, but his lack of style overshadows his substance. He made a case for getting a job in someone else’s administration.