New York City Mayor Bill de Blasio, armed with a $4.9 million campaign war chest and a double-digit polling lead over his nearest Democratic primary rival, is expected to cruise to victory on Tuesday, but not without a fight from four long-shot candidates.

The challengers — former City Councilman Sal Albanese, police-reform activist Robert Gangi, Brooklyn attorney Richard Bashner, and Mike Tolkin, founder of a tech startup — all have said they entered the race because they believe de Blasio has not delivered on his campaign promises to make the city more affordable and to close the gap between the city’s rich and poor that got him elected four years ago.

For months they have pounded at his record in candidate forums he did not attend, or on social media accounts that have far fewer followers than his, hoping to break through to voters who are historically less engaged in local primary elections. About 20 percent of the city’s 4 million registered voters showed up to vote in the 2013 primary election, city Board of Elections records show.

Albanese, 68, a councilman from 1983 to 1998 who lost to de Blasio in the crowded 2013 Democratic mayoral primary, acknowledged the uphill climb in unseating an incumbent. But he said he could not sit out the race when he had so many misgivings about the mayor’s leadership.

“I think people’s opinions of de Blasio has been etched in their minds by now, they really know what he stands for,” Albanese said. “My challenge is to convince them that I can do the job . . . I’m an underdog, but an underdog who can surprise people and win.”

Albanese emerged as the front-runner among this year’s longshot opponents after becoming the only other candidate to qualify for two televised debates against the mayor. He did so by raising the minimum of $174,000 needed to qualify for matching funds from the city Campaign Finance Board, and running a campaign debate sponsors deemed visible enough to merit time on the debate stage.

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He used the televised appearances to spotlight scandals that occurred in the mayor’s first term in office, including state and federal probes into de Blasio’s campaign fundraising tactics that focused on whether donors were giving preferential treatment by his administration.

Prosecutors concluded in March that while de Blasio’s aides broke no laws, his team came close to violating the “intent and spirit” of anti-corruption laws.

“We need a higher standard for mayor,” Albanese said at Wednesday’s debate. De Blasio defended his aides, saying “high standards were kept.”

A July 27 poll of Democratic voters conducted by Baruch College and NY1 news found that 57 percent would vote for de Blasio, and 3.5 percent were for Albanese. The remaining candidates were not tracked and no other polls have been conducted in the primary race.

De Blasio has raised $4.9 million since 2014, according to campaign finance records. Albanese is a distant second, raising $207,000 since opening his campaign account in December.

De Blasio recently launched a $2 million TV ad campaign. None of his challengers can afford airtime in the nation’s most expensive media market, so they have posted videos to their websites instead.

De Blasio campaign spokesman Dan Levitan said the mayor is not taking the race for granted and “is fighting for every vote across New York City.”

The other primary candidates say they will continue fighting for votes.

“Our intention, as unlikely as it would seem, was to win. We want to make these changes,” said Gangi, 73, who is calling for an overhaul of NYPD’s practices that he says disproportionately target African-Americans and Latinos.

“We want to educate as many in the public as we can reach, to expose racially biased policing in our policing system,” Gangi said.

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Tolkin, 32, a Brooklyn resident who was raised in Jericho, said as a business owner he would aim “to bring private-sector best practices” to City Hall to “address our number one challenge, which is economic inequality.”

Bashner, 57, a Brooklyn Community Board member, has tried to cast de Blasio as beholden to donors to his campaign. Bashner said he would push for changes in the city’s campaign finance laws to make it “easier” for other candidates to qualify for matching funds.

“We need a mayor elected by the people and not big money,” Bashner said in a video posted on his website.

The primary winner will face the Republican nominee Nicole Malliotakis, a state assemblywoman from Staten Island who avoided a primary after other GOP candidates bowed out of the race this summer.